Why do Christians rarely ever mention about eating Shellfish being an abomination...?

Because that wouldn't be as politically fun. Also christians aren't ready to give up their shellfish. In addition, they aren't threatened by people who eat shellfish...and, nobody these days believes eating shellfish is wrong because back in the bible days, it may have been dangerous, but it isn't today. Like shellfish may have been unsafe to eat at that time, but today they are safe. Christians fail to apply this rule to other "abominations" in the bible, however...