>It's simpler (which leads to progress)

I wouldn't say current diffusions models are simpler, in fact they are much more complex than even the most "complex" GAN architectures. However it's exactly because of all the other points that they have become this complex. A vanilla GAN would never be able to endure this much tweaking without mode collapse. Compare that to even the most basic score-based models, which are always stable.

Sometimes, the "It just works™" proposition is much more appealing than pipeline simplicity or speed.