Urien is the sole rep from the SF3 series, which was played for a long, long time during the fighting game drought before SF4 was released. It's a beloved game whether you like it or not, and Urien's moveset is quite varied, making for a potentially very interesting fight in a Megaman game. Whether you like it or not, he's the perfect choice.
Conversely, although Guile is a loved character, he is not a good choice because of his simplistic moveset. Aside from Supers/Ultras, he has Flash Kick and Sonic Boom. This would not make for a very fun fight, and would certainly leave not only Guile fans, but everyone dissapointed.