In the core set are 200+ player cards - so about 30 player cards per race - if there no neutral player cards. So it isn't a problem to add a new race - in the core set. But Tyranids are iconic and, in the tabletopgame, very strong. So many players love them....
It would be better - and I will get it, then tyranids are added. I think, similar to Necrons, they don't need allys.....