The existing official backup fight that was Stipe/Pav if Jon got injured...? They already had a fight line up for the exact scenario that happened before they decided to moronically pull Stipe and need someone for Pav to fight too instead of doing the backup plan.
And if not simply doing that then Volkov made more sense. He was coming off 3 straight finishes of Roz, Romonav, and Tai, which was a longer and better streak than Tom's 1 win streak over Tybura. He'd even only fought Tai like a month earlier so had recently prepped for a fight unlike Tom. He was a way more logical choice. Hell, he even got Pav next anyways. In before someone says Tom beat him before this streak as though that negates Volkov had the better current streak.
Tom got a very lucky break they weirdly gave him the shot off only beating Tybura. It was largely due to the shared narratives that Pav and Tom finish fights quickly and wouldn't last long. It was sold on that, not Tom being next behind Pav in the division when he really wasn't.
Nobody actually thinks Tom deserved a title shot for beating Tybura. If Jon hadn't gotten injured he was 1-2 fights away and was going to have to fight Gane, Pav, or rematch Blaydes or Volkov first. Tom ironically ended up in the situation he would've been in regardless if Jon never got injured.