I think it's twofold, see: Most tyranid "horde" units (termagants, hormogaunts, gargoyles, and possibly genestealers) went down in price. Gaunts with flesshborers used to be 6 pts, now they're 5. Hormogaunts used to be 12, now they're 8 or 10 (upgrades are almost mandatory). Gargs are now 6 etc... However now our Monstrous Creatures are much more expensive (trygons fetch 200 points a pop!)
our only anti-tank comes from Monstrous Creatures as well. Add onto this that tyranids need synapse to function, and now that warriors went up in price and die quicker, you'll soon find yourself failing leadership tests and screwing up your plans. I looove tyranids, but if you want a HORDE army, go for Orks, if you want semi-horde semi-elite, go for Tyranids/Guard.
Good luck!
DH