But we know that milk is important than meat. In Indian plains bufallo and cow are two main source of milk and meat. Bufallo is expensive to farm ( bufallo eats lots of food compared to cow and in hot climate they need lots of water compared to cow.)

As population grew society had to choose (as there was no commercial farming) which animal to use for milk and which animal is for both meat and milk.

May be because of ease of farming, color (cows are available in different colors,Bufallo is black only) so sort of racism help cow to get protection from religion. As there was no univeral law,but religion was universal so people decided to give cow protection under religion thats all I think. 

Fact1 : around 3000 years ago loads of wild buffalo were available in Indian plains.They even pose danger to human life so buffalo were considered not human friendly compared to cow So bufallo population was not in danger. so need no protection

Fact 2: Bufallo is mainly either swamp type or river type,so in those days if you dont have river or swamp nearby it is difficult to domesticate buffalo.whereas for cow no such restrictions, easy to have cow than buffalo.

Fact 3: Aryans who came from Europe didnt knew about bufallo ( buffalo is considered orignated either in China or in India. So it was easy for aryans to farm cow than buffalo.

Compromise: But society also ned animals for meat.lots of Indians they used buffalo for meat too. Sosociety can not ban both source of meat. Which to choose Cow or buffalo? for milk and both for milk and meat ?

Because of all facts and social behavior mentioned above I think society decided to give cow a religious protection.We all know Darwins theory "Survival of the fittest". Cow got the protection because at that time Cow was fit for it.

Now some hindus in sajha prefer to eat beef some hate it why so?

its all genetic I think

If the gene which is related to religion is strong in you then when someone talks about eating beef,the gene will initiate producing chemical which induce negative feeling so you hate.

If the gene which is related to religion is not strong inside you no such chemical will be released an you will feel comfortable eating beef.

Do we have gene related to religion ? Nope,but we can relate the gene which governs our social behavior to religion.