Trying to think of some possible factors from Sebs POV why he'd join Meeke;

Probably more chance of wins with Meeke.
Better pay and full season.
British driver (if that is even relevant)
And if Hayden was to get a full season at Msport, then Seb would be at a full manufacturer team vs a private outfit if he joined Meeke.
Relationship with Paddon might not be the best?

Reasons to stay with Paddon;

Already know each other and have built up an existing relationship. Would be easier choice to stay put.
Paddon is younger, so could have a lot longer in the sport if he can kick on.
Paddon doesn't crash as much as Meeke.


Purely speculative. Also it's possible Seb was going to be replaced further down the line by Paddon anyway, but I doubt it at this stage.