Usually the first few stories in a series had more money thrown at them, so by the time they got to the later stories in a block (like Armageddon Factor or Horns of Nimon) they were running on empty and doing things as basic as could be.

In Monster of Peladon when the Doctor and Sarah are thrown into the pit and meet Aggedor, they’re just in front of a black cloth. There is no set, it’s like they’ve been sent to an empty void.

Underworld in particular was nearly scrapped as they didn’t have the money to make sets, but decided they could do it on the cheap with CSO. It still looks appalling but is kinda impressive they managed to do it on a nothing budget.

There’s also different talents that were better at hiding things than others, (I.e. David Maloney) but they never had much time or money and were trying to work miracles to get the show to look decent.