Just Nature Being Nature
As soon as Scar takes over the Pride Lands, we see that the land isn’t as luscious and thriving with wildlife as it was when Mufasa was in control. So, we can only assume that Scar somehow drove the Pride Lands to the brink of destruction. Makes sense since, after all, Scar is a bad guy.
But let’s look at it through a more sensible lens. The start of The Lion King takes place sometime between spring and summer. When Scar takes control, that would be the start of the monsoon season, where there wouldn’t be any green in sight. It’s Mother Nature taking her course, not Scar being a failure of a leader, though that could also be true.