why did the pressure suits not compensate?
They didn't have any.
The design of the Soyuz and the spacesuits at the time was such that three crewmen could not all wear them and fit into the cabin and be able to actually move around to do anything. Disturbingly, with the same mentality that led directly to all three of the major NASA accidents (namely: we've done it this way for a while and nothing has happened yet, so it must be safe) they routinely allowed cosmonauts to go up without wearing space suits, though they may have had them to put on for EVA and the like. Soyuz did not require all cosmonauts to have pressure suits when EVA's occurred on the missions, because, with their modular design, the re-entry cabin could be sealed off while the forward module was depressurised and opened. This is in contrast to the NASA spacecraft that only had one cabin that was opened to space when the hatch was open, so all astronauts needed pressure suits if an EVA was to be performed.
The three main cockups were:
1: No pressure suits during re-entry.
2: Simultaneous detonation of explosive bolts that should have been sequential, which created a bigger jolt than expected, jarring open a valve.
3: The valve in question being located behind one of the seats, making it impossible for the cosmonauts to find and seal it in time. Had it been on the wall in front of them, for example, one of them could have put a gloved finger over the vent hole and plugged it, allowing them to survive re-entry.
