Mars is still flying, and he could even be damaged (even if he can regenerate, it could take longer depending on the damage he took)
But in case that he isn't currently flying:
-He fell deep into the sea and is still trying to get out of wherever he is.
-He landed far away from actual land, which PERHAPS makes it impossible for him to come back because he needs to land on proper ground to be summoned (i think it makes sense tho)
-If distance is a factor, he landed way too far to be summoned again (this option is the less likely one)
-idk if that makes sense but perhaps the Gorosei can only teleport themselves around together as 5, not being able to do it alone. That would explain why did Saturn came along with Kizaru and his fleet, when he could've simply teleported to Egghead from Mariejoa if he wanted to.
When he shows up again (if Oda cares to explain why didn't he came back to Egghead), then all that should be properly explained.