I have heard the Cruiser argument before. They were coasting down hill for a thrill and the Cruisers fit larger tires. They quickly dumped the heavy single speed cruiser frames for touring frames with 26 inch wheels laced to the multi speed hubs to get wider tires on them and gears to deal with the hills.

Its the same line as the BMX has it roots in the Stingray as bike thats really a kids sized cruiser.

70's Mountain bikes gemoetry was closer to Touring bikes when they first came out with the "Mountain bike" except they had 26inch wheels to make more room for wider tires primarily and flat bars. The Top tube length was close to the same as the seat tube. Now mountains tend to have long top tubes in relation to the seat tube to allow for climbing steep hills without slamming your knees into the bar.

Cross bikes are more of a road geometry with clearance for wider tires that are usualy 700c based and drop or dirt drops more like a road bike.