It'S mostly the lack of itemisation and wrong tuning for potential AP-brusier champions.
Champs like diana usually have to high ratio and to low bases to work on a low AP built, but lack the item options to get a suitable mix of AP and defense, while the high ratios make them great on full AP.
However, the biggest issue is where bruisers fail no matter if ap or ad, it's damage creep. Sustained combat patterns on a melee just don't work outside of a 1v1, you just don#t live long enough to do your job, so you need to burst down threats quickly. It's not just Ap-bruisers that are geared towards upfront damage at the current state of the game.
I've played many champs with bruiser builts back in the day, cause back then items didn#t scale as much in multiplicative ways. Nashors and guinsoos for example were stat sticks rather than haveing an additional ap-scaleing and amping other on-hit, so they were stronger on theri own and didn't scale that well with additional damage.
Squishys were actually vulnerable and you didn't need absurd damage numbers to threaten them, while even 2 tank items allready made quite an impact.
Now you feel like anything that isn't fulltank dies in a sec, and many champs feel like the hardly deal damage if they don't stack off items that synergies with each other.
There are to few items that are strong on their own, to much dependency with other items so you usually get somewhat foced into certain builts.
The only exeption for that are trinity, bc and ravenous aswell as to some extend titanic and steraks(though both need additional health, but that's still a big pool of items to choose from), which are all ad-bruiser items and the reason why ad-bruisers still work to some extend.
Also, Riot never really released an proper AP bruiser, it's been usually either tanks with high enough ratios to make use of some AP items, or AP assasins going more tanky, and everone looses their shit when AP assasins go more tanky(fizz/ekko and even release sylas who arguably was designed as AP-bruiser and now gets turned into an assasin)
P.S. I also like The DJinn's base AP idea, haveing some overall damage scaleing per level and potential bruiser items that give a % base AP increase like sterkas does for AD would also help alot.