since nobody has given an honest answer without the word asshole and dick jokes sprinkled about...

in the 80's we started seeing a lot of emissions regulations starting to pop up and that combined with a few other things led to the death of the muscle car era. emissions control devices have historically been terrible in terms of efficiency of a system, maintenance intervals and severe performance loss. while technology has progressed significantly since then, regulations have also tightened. while modern cars have become an order of magnitude more powerful and efficient, there are still massive engine life and performance gains to be had by removing these systems.

some genuinely are just assholes, but by and large these modifications are done to increase power output and to prevent certain parts from failing that would otherwise electronically keep the engine from running