Why Muslim woman has to get married with only Muslim man? And if she loves Buddhist or Christian man and get married with him it is a big sin? As known many Muslim men did not follow Islam at all and they have bad character so what is the difference between them and not Muslim men?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The Marriage of a Muslim woman to a non-Muslim man is not allowed in Islam. If she gets married to a non-Muslim the marriage is null. If she knows that this matter is prohibited, then she is an adulteress sinner, hence she must repent from this sin.

We have already mentioned in a previous Fatwa the prohibition of marriage of a Muslim woman to a non-Muslim man, while a Muslim man can marry a woman from the people of the Book. For details read Fatwa: 83101.

If you mean by "many Muslim men do not follow Islam…" that they practice what entails disbelief then they are disbelievers, if evidence has been established against them, even if they claim themselves to be Muslims. On the other hand, if you mean that they commit sins, which do not entail disbelief such as drinking alcohol and the like, or they are of bad character, then they are not disbelievers. A Muslim and a disbeliever cannot be equal by any means.  Allaah says (which means): {Then will We treat the Muslims like the criminals? What is [the matter] with you? How do you judge?}[Qur'an 68:35].

The "criminals" in the verse refers to the disbelievers. The difference between a Muslim and a disbeliever is the religion of Islam by which one enters Paradise in the Hereafter. A disbeliever rejected Islam in this life, so he will remain in Hellfire forever. How can one make Muslims and disbelievers equal while Allaah has distinguished between them?

Finally, Islam encourages a Muslim woman to marry the one who is religious and of noble character as it is proved in sound prophetic narrations.

Allaah knows best.