I think it was both actually - too lightly armored and the funky missile.
Missile system never worked as advertised, but to accommodate the missile, you needed to mount a 152mm main gun, which was a bit large for the size of the chassis
Tank had to be lighter to airdrop, which meant lighter armor. But too many planners (esp. the 82nd light-infantry-hooah-airborne-ranger-uber-alles brain trust) saw "turret + big gun + treads" and somehow came up with "just as good as any other tank" and ended up trying to use it in ways it shouldn't've been...