Lots of reasons.
They are already overwhelmed with production of (mostly) Marvel figures - Marvel movies & TV shows are coming fast and furious from that one license, so it is hard to keep up. No one will have a Spiderman NWH figure until 2023.
There are no current A & P films - so if there's a pecking order for licenses, these are over priorities.
Also, NECA has posted recently that approval process is slower recently (I assume since Disney bought Fox), and NECA's A & P business has always been steady, so even if HT is planning new A & P figs, approval may be taking longer.
Finally, I'm not a huge fan of either franchise, but I think I heard there's an Alien TV series coming. If that's true, a lot of time licensors will hit pause on making stuff from older films so there's no competing product for new stuff.