Because people are always trying to persuade or encourage people to be apart of their religion. They think their religion is the "right" one. People have different beliefs about things & those beliefs should be respected. However, those beliefs should not be forced or shoved into someone else's face.
It has not "become" that way- it has always been that way. Religion is inherently divisive- the vast majority of religions after all try to keep their adherents by assuring them that theirs is the only "true" way and anyone who doesn't believe it is immoral, evil, hateful, "sinful", and so on. When your religion promotes the idea of prejudice against other groups and paints them as "less than", of course the result will be hate.
The reason why religion is so full of hate is because they are failing to follow Jesus' command at John 13:35 "By this all will know that you are my disciples—if you have love among yourselves.” Love is the identifying mark of a true Christian, and it should be the most apparent quality in religion.
I have found that many people are quite concerned about the lack of love in the world. Sounds like I am not the only one. Why do you think this is the trend? Actually the Bible foretold this situation! "You may as well know this. Timothy, that in the last days it is going to be very difficult to be a Christisn. For people will love only themselves and their money; they will be proud and boastful, sneering at God, disobedient to their parents, ungrateful to them, and thoroughly bad. They will be hardheaded and never give in to others; they will be constant liars and troublemakers and will think nothing of immorality. They will be rough and cruel, and sneer at those who try to be good. They will betray their friends; they will be hotheaded, puffed up with pride, and prefer good times to worshipping God. They will go to church, yes, but they won't believe anything they hear. Don't be taken in by people like that." ( 2 Timothy 3:1-5) Living Bible It also explains the reason for it. "He who does not love does not know God; for God is love." ( 1 John 4:8) Revised Standard Bible. People are not following what Jesus taught, Church's are not teaching what Jesus taught. Answer for the person that asked the question please.
Religion, Christianity, is only now barely starting to be tolerant of others that are different. But only because they are being forced to. Racism, religious persecution, slavery, anti gay sentiments ect are all due to Christian religious beliefs.