I think gp got it the wrong way round. Single front rings came in because front mechs were always a reliability weak spot as unlike the rear they must shift chain under tension. Also if you ditch them you can run narrow wide chain rings which hold the chain better on rough ground, and you get more clearance under the bottom bracket.

After we realized we didn't need 30 gears, new suspension designs also became possible.

You're right that rear mechs frequently die though.