The problem with vanilla is that gear in and even in BWL was a very, very small power creep. You could learn the bosses better and that would help to prevent mistakes, but in terms of actual speed, gearing the raid better didn't help in an exponential manner. If MC took, say, 60 minutes to clear, gearing the raid out in tip top gear might reduce that to 40 minutes. Flasking the entire raid, and using some sort of onyxia buff or something, however, would cut that time down to 20 minutes. Flasks are such a massive buff compared to the actual gear that not using them in many cases doubles the amount of time you spend raiding. It is not a "small" amount.
This is not the case in today's current game, but for Vanilla, those alchemy buffs were a pretty darn big deal. Tthe reason everyone is flasking is because it makes the farming portion of the game go significantly faster, which is arguably the most boring part of the game and is the most soul sucking. Any way to reduce that portion of the game is a big deal. I think in Classic, people just didn't have this mindset, as I recall what an insane hassle it was just to get an entire raid of 40 to flask, and then what a crazy hassle it was to farm multple flasks each week. Raiding was insanely time consuming and expensive back then due to this. Again as you say, it's probably gotten better these days just simply because people know what to do.