Simply put, Christopher didn't have the experience to be boss. Too hot-headed, and many times he came up short against older guys: Paulie, Phil Leotardo, Richie Aprile, etc. Had he been older, maybe, though he still lacked the attributes required for leadership, and his constant drug abuse and weak self-discipline (without Ade and Tony's guidance he was basically a lost cause) probably wouldn't have taken him far. I also don't think enough people respected Chrissy.

Premature, rather unfortunate deaths aside, I actually think Gigi Cestone might have had the fittings to be boss - perhaps not all the fittings, but certainly the decision-making (the Mustang Sally hit), the swift assertiveness (killing "Spoons"), the appropriate people skills (handling Ralphie and Pat Parisi) and he had the required respect (by all bar Ralphie, though Ralphie didn't really respect anyone, so that's no biggie). He wasn't vocal until he needed to be, too - something that Chrissy never had.

But of course, becoming boss brings all sorts of pressures - look at Silvio.