Actually, MTBs in the late 80s-early 90s used thicker gauge tubes, typically straight - not butted. The ride was on the wooden side - not lively - at least on the mass produced bikes. Of course, front shocks didn't exist then, so the frames had to be able to take the shock of rough trails (actual MTB trails were few and far between then, too). I'm talking about true single track/no track, not fire road riding such as the lighter, parallel tubed bikes were designed for.

Riv uses butted tubes, lighter gauge (though not lightweight) depending on use. The bigger the intended load, the heavier gauge the tubing.

It wasn't until front shocks became developed that lighter tubing came into vogue. By then frames were migrating to aluminum and lighter weight hardtails led the way.