Because diet is an entirely personal decision for the individual, based upon the conviction of their conscience, according to the New Testament.
The attitude is one of tolerance for the "
conscience" of each individual believer:
Welcome a man whose faith is weak, but not with the idea of arguing over his scruples. One man believes that he may eat anything, another man, without this strong conviction, is a vegetarian. The meat-eater should not despise the vegetarian, nor should the vegetarian condemn the meat-eater.
Again, one man thinks some days of more importance than others. Another man considers them all alike. Let every one be definite in his own convictions. If a man specially observes one particular day, he does so “to God”. The man who eats, eats “to God”, for he thanks God for the food. The man who fasts also does it “to God”, for he thanks God for the benefits of fasting. The faith you have, have as your own conviction before God. Let us therefore stop turning critical eyes on one another. If we must be critical, let us be critical of our own conduct and see that we do nothing to make a brother stumble or fall.
We should be willing to be both vegetarians and teetotallers if by doing otherwise we should impede a brother’s progress in faith. Your personal convictions are a matter of faith between yourself and God, and you are happy if you have no qualms about what you allow yourself to eat. Yet if a man eats meat with an uneasy conscience about it, you may be sure he is wrong to do so. For his action does not spring from his faith, and when we act apart from our faith we sin.
(Romans 14:1-23)
So, from the beginning, Christianity was tolerant about dietary matters but emphasized that every individual Christian had to live in accordance with their own conscience:
if you feel guilty about eating something, then it's sinful for you to do so but don't impose your personal conviction on others, was the advice from Paul.
IMHO there is no logical, ethical rationale for abstaining from pork out of compliance with some antiquated custom of the ancient Israelites, while eating meat in general. Being a vegetarian makes sense to me (i.e. animal welfare) but ritual concerns about the "
impurity" of certain foods just doesn't float my boat.
The only reason the Torah introduced these dietary rules was to distinguish the Hebrews from surrounding societies, helping to give them an independent, national-ethno-religious identity as a people set apart by God for His chosen purpose.
For Jews to continue abiding by the laws of kashrut out of respect for their heritage, and for Muslims as well vis-a-vis halal, is perfectly fine - but no one else is bound in conscience by these rules unless they want to be.
It's all about having the freedom to choose for oneself as an autonomous agent.