How can we explain the existence of some people who were born Muslims and others who expended a great deal of effort to achieve that?

Firstly: 

One of the things concerning which there can be no doubt is
that the greatest blessing that Allah bestows upon His slave is to cause him
to be born to Muslim parents, to live as a Muslim and to die as a Muslim.
This is by the grace of Allah, may He be exalted. This is a blessing from
Allah, as the one who is so blessed does not need to seek through different
religions to find the true religion that Allah loves and is pleased with,
and does not accept any other religion. The individual has not done anything
to attain this favour; rather it is pure grace from Allah his Lord, may He
be exalted. 

But this does not mean that everyone who is born to Muslim
parents understands the extent of this blessing and acts in gratitude for
it. Rather we see many, many people who are ungrateful for this blessing and
do not benefit at all from the fact that from the moment they opened their
eyes on this world, they saw Islam and the mosque, and the first thing they
heard was the word of Tawheed. Hence these people go astray and follow paths
of misguidance and kufr, and they are Muslim in nothing but name, and some
of those who are seeking for the true religion may be better than them –
although it is not correct to make such comparisons –when they come to know
that Islam is the religion of Allah and He does not accept any other
religion from His slaves, and they read and ponder until they enter this
religion with conviction and adhere strongly to its rulings. 

Hence the matter is nothing more than additional grace that
Allah, may He be exalted, bestows upon whomever He wills among His slaves
that they should be born Muslim. However He does not wrong others, as He has
made clear to them the paths of guidance and misguidance, and has shown them
His straight path and enjoined them to follow it, and has shown them the
ways of the Shaytaan and warned them against following them. And you see
some of those who were born to Muslim parents yet they go astray, and you
see some of those who were born to kaafir parents following true guidance. 

One of the clear examples concerning this matter which will
dispel your confusion is what you know about the situation of some of the
Messengers and their call and the response to it. We see Nooh (peace be upon
him); what did it benefit his son and his wife, when he was the Prophet sent
by Allah who called them night and day, but they did not respond and they
died in kufr. What did it benefit the father of Ibraaheem (peace be upon
him) when his son persisted in calling him, advising him and exhorting him,
but he rejected that and even played a role in throwing him into the fire?
What did it benefit the wife of Loot, even though she was the wife of a
Prophet sent by Allah and he called her to the truth, but she insisted on
following kufr? Yet at the same time we see Pharaoh, who was one of the
greatest tyrants on earth, yet his wife declared her Islam and disbelieved
in her husband. This is how human beings are; either Allah bestows upon some
of them the blessing of Islam, and they are grateful for it and strive to
adhere to it, and some are deprived of this blessing but they put a great
deal of effort into seeking until they arrive at the truth that Allah, may
He be exalted, revealed and commanded His slaves to follow His path. And
both parties must always say (interpretation of the meaning): “All the
praises and thanks be to Allah, Who has guided us to this, never could we
have found guidance, were it not that Allah had guided us” [al-A ‘raaf 7:43],
and say as the Companions of Muhammad (blessings and peace of Allah be upon
him) used to say: By Allah, were it not for Allah, we would not have been
guided and we would not have given charity or prayed. 

For the one who used to follow a different religion,
believing in it and following its Prophet, then Allah, may He be exalted,
guided him to the light of Islam and he believed in the Seal of the
Prophets, the glad tidings from the Prophet (blessings and peace of Allah be
upon him) are sufficient for him: 

Al-Bukhaari (3011) and Muslim (154) narrated from Abu Moosa
al-Ash‘ari (may Allah be pleased with him) that the Prophet (blessings and
peace of Allah be upon him) said:

“There are three who will be given a double reward: a man who
had a slave woman whom he taught and taught her well, then he set her free
and married her – he will have a double reward; a believer from among the
people of the Book who believed (in his Prophet), then he believed in the
Prophet (blessings and peace of Allah be upon him) and followed him and
believed in him– he will have a double reward; and a slave who fulfils his
duty towards Allaah and towards his master – he will have a double reward.” 

See also the answer to question no.
11783 and
150930 

Secondly: 

With regard to what has happened between you and your parents
of differences in culture and religion, this is the same thing that happens
among human beings. What has happened to them of divorce is something that
happens frequently, especially with repeated problems that create serious
friction between them. 

What we advise you to do is to focus primarily on telling
them about the religion of Islam and calling them to it. If Allah blesses
you by guiding them, then there is the strong hope that they may be
reconciled, in sha Allah. If it is decreed that they should remain
separated, they will have gained the true religion and will be guaranteed
salvation from the punishment of Allah. 

In fact this is one of the greatest rights that they have
over you, that you should make them share this blessing that you enjoy, the
blessing of Islam and submission to Allah, the Lord of the Worlds, and
worshipping Him alone with no partner or associate. So do not spare any
effort in this matter, because it is an important mission, and you will see
its good effects, if Allah wills, in this world before the Hereafter. Seek
help in that by offering supplication (du‘aa’) for them whilst striving to
take practical measures in support of that mission. You can also seek help
from experienced people in your city to achieve that. We ask Allah to guide
you and help you, and to accept your righteous deeds and guide your parents
to that which will bring happiness in this world and in the Hereafter. 

And Allah knows best.