Perhaps there just isn't enough of a market for halal beef fat to make it worth the while of the carcass processing plants. They might sell it to be mixed with other beef fat.

I can think of another reason too: these chippie vendors might also want to cater to other people who don’t want to eat beef e.g. some Hindus. Some halal Indian restaurants in San Francisco do something similar: they serve halal chicken and lamb, but no beef.

tl;dr Vegetable oil ticks boxes for multiple sets of clientele.

That makes a good deal of sense. I'm a vegetarian, I appreciate it, but was merely surprised given that halal beef and halal beef fat exist. Thanks for these further speculations.