Why were Bennett, Nimoy, and Meyer able to create better films with less money involved? My answer: because they were better filmmakers.
ST2 & 3 were shot on sets. ST4 had very few visual effects. And the ones they did have were completed by ILM. Shatner has already explained the major effects houses were booked on other projects. Did he and Bennett select a bad one, yes. But it was not their first choice. And if memory serves, most of the original sets were destroyed, damaged or altered for TNG. So they had to create a new bridge, shuttle bay, lounge, ect. The corridors were from TNG.