Luke: She's rich.
Han Solo: [interested] Rich?
Luke: Rich, powerful. Listen, if you were to rescue her, the reward would be...
Han Solo: What?
Luke: Well, more wealth than you can imagine!
Han Solo: I don't know, I can imagine quite a bit.
Aaaand... the cynical, hard-bitten, seen-all-sorts-of-treachery, experienced smuggler Han Solo conveniently fails to notice that Princess of just-blown-to-bits Alderaan is no longer is powerful or rich, being that her "fiefdom" got destroyed and the Rebellion is dirt poor (which he knows quite well, after his ex-gf swindled him and his smuggler buddies out of money to fund that same Rebellion).
Is there any canon explanation for why he got conned by a naif farmer kid?
An obvious explanation could be that he didn't know Leia was a princess of Alderaan... but she wasn't a nobody Princess Vespa - she was an Imperial Senator.