Because different environments usually had to be "tuned" for. Things like elevation, air density , humidity, air-fuel ratios etc all had to be fine tuned for each race location.

Now...that doesn't exist. This, alongside electrical motors having little variantion in a highly regulated sport ( racing leagues have strict limits to hardware and design to help keep the the racers equipment about the same, emphasizing driving skill over hardware ).

It weakens the competitive aspect, though doesn't "kill" it outright