I’m not an expert (or even well versed) in the BSG canon. Having said that, I’ll try deduction and logic (for a change).

“MkII” implies there was a “MkI”. It would have been an improvement over the MkI (obviously). At the opening of the series, the MkVII was in service. It was five variants later and one can assume that many improvements had been incorporated into the latest variant. One of those improvements is why the MkVII failed at its task. Networking. The net work was corrupted at the start of open hostilities and the MkVII became target practice for Cylon Raiders (or pretty much anything that was armed) because the corrupted network shut them all down. After launch. Grim. And before I forget, everything on the network crashed and that included the latest Battlestars.

So why the MkII?

At the start of the show, the Battlestar Gallactica was the last of the Battlestars and was being converted into a museum of the Cylon War. Gallactica wasn’t part of the network, either, sharing the timing of having been designed and built before the network was. During service, the Gallactica was equipped with MkII Vipers and a compliment of them were aboard the Gallactica when hostilities commenced.

Clearly neither the Gallactica nor the MkII Vipers had been de-milled for display. Fuel them, arm them, launch them, all the MkII Vipers were evidently still fully operational (or could be made operational quickly). The Gallactica itself was also not de-milled. All sensors, armament, launch and retrieval operations, were still equipped and operational.

Lucky for the Colonies because anything networked no longer functioned.

Seeing the MkII in action, the ship was durable enough to satisfy Grumman. It was just the parachute the Colonies needed to stop the tactical free-fall and was on-hand. Kick the tires, top up the fluids, and arm the guns. It was ready.