... because 'long' does not collocate with 'later'. You could however say 'much later' or 'many hours/ days/ weeks/ years etc. later'.

That's a great question. You can use similar words with "later", for example you can say "far later" "way later" "a long time later" "much later" So why can't you say "long later"? Well, who says you can't? It is not wrong, but most people don't say it that way. That makes you a poet.