The British have been trying to replace that rifled gun with the German L/55 smoothbore, too, but they don't have the money. It wasn't clear at the time the tank was designed, but the rifled barrel turns out to have been a mistake. The round for which it was primarily designed (High Explosive Squash Head, or HESH) hit a sort of technical dead end with the introduction of spall liners and layered composites in tank armor.

Read the comment by Steven Den Beste, who explains the situation pretty well (aside from getting the HESH acronym wrong):

http://www.professorbainbridge.com/professorbainbridgecom/20...

True, but the smooth-bore guns used in most other tanks all fire fin-stabilised rounds.