You could 40-man raid UBRS and Strat but it was not the norm. I can't remember ever seeing more than 10-man (and very rarely 15) groups in Strat. UBRS did have some bigger runs, but that was still 10 to 20. Not 40. Many did 5-man Strat and 10-man UBRS.
MC was easily cleared if you had the Fire Resist gear and enough decent players - it didn't require much dps. The difficulty was down to all the things others have mentioned - especially the bad/lazy/ultracasual players.
I remember when we started using dps meter addons in my guild, and found that one hunter was only auto-attacking (with maybe a few abilities once in a while, like 3 arcane shot or similar). When asked why, she mentioned that typing in chat made ability use impossible. We had an AQ40 run with an almost naked priest, who had sold his gear during a ragequit and just wore a few pieces of grey and green gear in some slots. Tons of players had insanely bad builds and gear sets. So much "prestige" gear had awful stats. To be an above average player you didn't have to do much more than not slack, enchant your gear and have reasonable gear/talents. Casual players today min/max and have rotations more optimal than even good players back then.