He still owes Jabba money, which means a long line of bounty hunters trying to collect the price on his head no matter where you go. There’s something to be said for reducing the number of people actively trying to kill you personally.
I don’t disagree, but then if he was a committed Rebel, why:
A) aren’t some of the Rebels contacts willing/able to pass on his payment of debt to Jabba while he continues to serve them, especially if he’s so valuable to the cause; or,
B) are the Rebels not at least sending some back-up or company with him considering how much they owe him after Yavin (Luke would be more than willing to name one); or
C) is Leia treating it like she might probably will not see him again (for some time) if he leaves;
…if he actually is a committed member of the Alliance.
None of these, Leia most notably, and the afore mentioned interactions with the Rebels make sense taken all together, to me, assuming Han is a committed Alliance member at the start of ESB.
It makes sense to me that Han would need to go and pay off Jabba solo, if he is still riding the line between committing to a higher purpose and his cynicism, still relying entirely on himself and not opening up to his friends. His relationship with Leia is both a metaphor for and the driving force of this balancing act.
Which has a fantastic pay off at the end of Empire, and in my mind is rounded out partially in Luke and Leia rescuing him in ROTJ but is never fully resolved, disappointingly because he has so little to do in the ROTJ screenplay as it stands, when all is said and done.