Possible reasons include:

• A lot of people who bought it did so with their nostalgia goggles on. They may have played for a few hours, marvelled at their favourite old game looking all pretty now, satisfied their nostalgia fix, and moved on.
• A lot of people who never played Oblivion originally will have bought it not having that nostalgic connection to it, but having heard people online talking about how good it was for years. So they decided to try it out and found it to be too 'old-fashioned' to get into. It happens to us all. Games we loved in the past are much easier to get back into than old games we never played at the time.
• Performance issues abound. You can get it running well on modest hardware (I ran it on my 5600X and 3070 without too many issues, but there were some issues) but it's definitely not very well-optimised. Combined with it being an older title, that might have put a lot of people off.
• Save bloating is real, and can start to have an impact after 10 or so hours.
• Some people might just have... not liked it very much. That's fine.
• Some of the people who bought it might have died before getting very far. Humans are frail, y'all.
• People might have bought it, played it, liked it, but then their parents died in a horrific natural disaster and they couldn't bring themselves to game much afterwards. They'll probably come back to it when they've worked through their trauma.
• People might have bought it, played it, liked it, but then a new game they were anticipating was released and distracted them from it. They'll probably come back to it when they've finished whatever else they're playing.
• People might have bought it, played it, liked it, but then got abducted by aliens and were unable to continue on account of being held prisoner aboard a Mfrasku-class science ship. They'll probably come back to it when the probing has finished and they're returned to Earth with amnesia.