You have to look at what the 'rule-breaking' alliances have/had in common.
NPO were huge, and with their treaty web were virtually untouchable. It took an unheard of level of co-operation among pretty much the rest of Planet Bob before they were beaten
DBDC are essentially all the top non-neutral nations, and are thus virtually untouchable.
Why do/did these guys break the 'rules'? Because they rarely have to suffer the consequences. It is usually easier for the rest of us to just let them get on with it than it is to oppose them and win.
Why does everybody else follow the 'rules' - because if an alliance the size of TTK flouted all the rules we'd lose all our friends and be curb-stomped repeatedly until we learnt our lesson. And with the current bloc set-up, there isn't really a clear-cut side which is overwhelmingly superior enough to get away with it.
tl;dr - 'Might is Right'