James had always been keen on the idea of a catholic bride for Charles:
a) Parliament vehemently opposed a catholic marriage, so it was a way of showing who was boss!
b) He could play the role of "Peace maker" in Europe. He would bring Protestants and Catholics together, stop the war and everyone would applaud him. The war was causing problems in England too, as trade with the continent slumped.
Charles' and Buckinghams visit to Spain in 1623 ruined this plan - they were so annoyed that they became determined to make war against Spain. They even sent a fleet to Cadiz to re-singe the King of Spain's beard - it was a failure. This was why Buckingham thought the French marriage was a good idea.
A Protestant marriage would have looked as if Parliament had some say in the matter.
As Ilyala says, Buckingham soon changed his tack and fell out with the French, deciding it would be a good idea to help the French Protestants in La Rochelle (also a failure).