Well, the noble buff could use some changes, it's OP the first few levels but drops hard later with vayne and fiora beeing really weak individual pieces and garen after the nerfs only beeing decent.
Even if you get all 6 nobles, leona is not great for a tier 4 and Lucian only is strong with items while kayle also feels a bit meh for a tier 5.
This tricks many into thinking vayne and noble are strong cause of how they perform early, especially paired with garen and knights that also are strong early and drop hard later on.
Another issue is that many other options are simply a bit to weak or just lack the early pieces. Gunslingers/pirates is decent and easy to get, and the pirate boon accelerates your econ alot so it's never a bad idea even if you loose in the early, while noble goes well with knights which is the other easy to get strong early perk.
Sorc often have issues getting theor comp together early, bladmasters need help(especially fiora), assasins aren't to great early and also often struggle getting a comp going, void buff sucks cause armor is nearly non existent if you don't face guardians or noble, and reksai needs buffs, shapeshifters are very dependent on getting lvl 2 pieces so they have a chance to shift befor dieing, and even then one of their early pieces is elise which could easily be the worst peice in the game, and then we have braum who is very underwhelming and needs a teir 4 for his perk.
So yea, it's no wonder that vayne+knights/noble and gunslingers/pirates are the most common comb, as they are the most reliable to get with the best earlygame(or save gold gain in case of pirates)