It probably has more to do with a shortage of wingers, than it has a specific wish to keep Candreva.
I hate the guy as much as everyone else, but with Perisic publicly expressing his wish to leave - and the club doing most everything to grant him that wish, we'd only have Keita and Politano for the wings for 3 competitions, should we offload Candreva. Carrasco proably isn't coming, and we can't recall Karamoh (as far as I know) - so what other options are there? If we had a realistic alternative, I'd be all for getting rid of both, but it just doesn't look likely.
I know what looks best on the books also - if the club can get 40 mil. for Perisic, we're looking at quite a big profit FFP-wise (without being an expert), whilst what would most likely be some kind of measly loan + option for Candreva whom we bought for 20 million if I recall, doesn't sound quite as enticing.
There's nothing saying we can't just move him on in the summer, when there are more options in the market. His value likely won't decrease in any significant way anyway.
I know Candreva adds next to nothing, but I'm actually worried that Perisic, if he plays matches and sets his mind to it, could try to be an active detriment to the team. The motivation is just simply gone. That's much worse for the team as a whole than having someone who's just plain awful.
I'm not happy about it, but I very much understand where management is coming from. Keita + Politano, with Candreva as a reserve for the rest of the season looks very lackluster.
But just Keita + Politano could be outright catastrophic.