The problem with the tankette concept isn't so much that they're so wee ickle, it's that they're still heavy, both in mass and required maintenance. As others have said, they were also barely armored and very vulnerable to anti-tank rifles and even certain heavy machine guns. The US practice of sticking an M2HB on anything that could take the recoil - and a few things that really couldn't - was the final nail in their coffin. Plenty of Japanese tankettes were destroyed by USMC .50-caliber rounds.
Mmm, the Ma Deuce. We're going to be using those for another century, I'm sure.