Really? Several? I've been here too. Like I said before, you put up a link to a thread asking for better monster creation rules, and I'll put up three asking for more lore and we'll see who runs out first.
The problem with the 3e rules was that they were just far, far too complicated. It took a LONG time to make a higher CR monster in 3e. Caster? It could easily take an hour or more to create a single monster. That was the problem. So, we got streamlined, simple rules for monster creation in 4e that worked really, really well.
But, because they were too transparent and the math was too regular, things were deemed to be too predictable. So, we get 5e which is all wibbly wobbly as far as monster design goes. Where you have monsters with similar CR's and WILDLY varying complexity. Meaning that the monster creation rules were largely opaque and very hard to use.
Even things like Sly Flourish's Lazy DM's monster creation rules are freaking massive. Forge of Foes is a HUNDRED AND TWENTY EIGHT PAGES long.