Because when he won belt from Adesanya, he was everywhere in the media. Starting from interview, ending with his past controversial statements. Before fights with Adesanya, Pereira or against top of middleweight division, did many know about him? He was "that redneck who talks crap" only. But he has been in UFC for 7-8 years before that. Everyone remember champions, nobody remember losers. Everyone gonna pay all attention to Izzy and DDP, and I am afraid that Strickland will be lost among rest of UFC fighters.
First of all, he was holding back at the beginning of his career and wasn't very "controversial". Second of all, it's pretty normal not to be famous when you're low in rankings or not ranked at all. But the point is, once you get famous and people recognise you - you can't just switch it off.
It's not like everyone will get a sudden amnesia and not know who Sean Strickland is just because he no longer holds the belt.
All he needs to do to stay hyped up, is to perform somewhat OK, win a fight once in a while and keep doing what he's doing. Look at Nate Diaz (just one of many examples), he had never won any belt and was losing a lot of fights, but didn't stop him from being one of the most famous UFC fighters. Strickland has way better potential than Nate. After that press conference when he bashed that canadian reporter, he got way more exposure in social media than after winning with Adesanya. He got tonnes of quotes from people not even remotely interested in mma.