Because they have to trade someone, either to make room for Arano or to increase the bullpen's optionality. Unless they waive or trade one of the lefties or the injuries take care of it for now.
All the off-season reporting just seemed to lean that way, suggesting they prefer Hunter to Neshek (he can give you more innings), or they just think Neshek is easier to trade. As much flak as Hunter took he put up twice as much WAR (albeit mostly because of IPs) with basically the same FIP and WHIP, and is younger.
My feeling is that given Neshek's age and history, better to trade him before he breaks or ends up having a bad year, rather than waiting to see what he can do. OTOH he only pitched 21 innings last year so maybe he'll be stronger this year.
It's mostly about money, Phillies would probably have to eat $4.5 million to trade Nicaso or Hunter, with Neris maybe only $2 million. Depending on return.
Some teams may also like the fact that Neshek is controllable though I doubt anyone wants to pay him $7 million in 2020 (especially given the way the market has changed even since he signed that deal).