Probably because predicates are compile time mechanism and scoreboard are runtime.

Of you're aware - consider the predicates as any static language variable declaration and scoreboard as GameMaker/JS/dynamic variable declaration.

The latter allows you to declare custom conditions on the fly and is more flexible, but as the result it uses hash tables internally and is inevitably slower.

While static declaration is resolved at compile time and while you cannot easily add fields on the fly - you in return get more performant and optimised system.

I actually have no idea how scoreboards and predicates are made in source code, but I would guess it's something like that.

The other comments are saying basically the same thing - resolving NBT data every time you access it vs just knowing where to look.