Selective religious practises

This post might offend some. As I am not here to please anyone, it shouldn't matter.

Within my limited travels, from my young adulthood till now, I have eaten at various eateries, from street food in Siem Reap to (almost) fine dining in Kuala Lumpur, but I failed to understand a few religious practises related to food.

I failed to understand why some Hindus don't eat beef, but eat other meat. It's clearly stated in various Vedic scriptures that Hindus must be vegetarians. Some where in the past 4000+ years, this ancient religion's practise has changed. Now, some don't eat beef & pork. Some don't eat beef, but eat pork. Some don't give a damn.

I failed to understand the importance of halalness of the meat to a Muslim, but orders a glass of red wine to accompany his medium rare fillet mignon. Forgive me if I am wrong, but doesn't the Quran prohibit the consumption of alcohol?

Some have selective memories, but some selective religious practises, I suppose.

What do you not eat?