Actually, I would say that the reason why shield breaks in melee rarely occur is due to shield poking being so prevalent and the fact that you honestly have to aim your shield vs certain moves. I mean I know this is a bottom tier character but take a look at Game and Watch's shield and a few others. They are ridiculously bad, I mean a full GaW shield (normal, not light shield) does not cover his entire body.It would suck though if shield were really good ... I mean they're called fighting games not blocking games. Also it is lame how in Melee/Brawl shield breaks basically never happen.
Also obligatory "shield is good if you know how to use it"
In terms of brawl, it is probably due to shield stun and how slow characters are. The only character I ever see shield break in tournaments when i used to play brawl were Snake and Peach (+ the random shield break marth.) Snake specifically due to f-tilt and grenade having massive shield stun) and Peach due to d-air/glide toss turnip pressure
If shields properly well shielded like in 64, I could image shield breaks being pretty common in melee, at least in spacies matches.