Many, many reasons
1) Players and the recent culture surrounding warhammer has been emphasizing laziness and everything being made into a product you can purchase and quickly set up for quick games. Speed painting, three color minimum, hyper elite armies, no narrative, hyper simplified rules, etc. This has also effected terrain, the cheaper and quicker the better is the mindset for many people. It is why a lot of these dogshit tables are just unpainted snap together MDF or cardboard sets likely bought online in a bundle for under $100
2) No more tutorials. Remember when GW used to have terrain tutorials in almost every single white dwarf? Remember when every single army book had a section containing ideas and tutorials for making terrain for your faction? It is a rare day when that happens in the modern times
3) Rules themselves. The rules in 40k and sigmar are terribly written and many players feel bound to them for various reasons (aforementioned laziness, social anxiety, esports vidya mindsets). Due to this, many players only play with ruins terrain since that is the only terrain type in the rules that actually block line of sight, and only play with the official competitive terrain layouts.