vegetables in general and tasting bad is moreso lack of preparation/cooking rather than the actual thing itself most of the time. Brusselsprouts is the polarizing one where its seen the most.
It's because traditionally bitter vegetables have been selectively bred to taste better. The brussel sprouts and broccoli your parents had were very different than what we have today.
Honestly I'm still pretty skeptical of this factoid. The Brussel sprouts now taste pretty similar to the ones I had in the 80s and 90s when cooked the same way. The whole "Brussel sprouts taste new" feels like some industry marketing to me.
Your memory of what something tasted like 30 years ago probably isn't super accurate. It's a fact that they've been selectively bred over the last few decades to taste better.
I mean wouldn't that also apply to everyone who thinks they taste better? And why would they have only started trying to make them taste good recently?
If you've been eating broccoli throughout the whole selective breeding process, then the flavor change would have been subtle enough that you don't realize there's been a change at all. If you ate them side by side, the difference would be noticeable.
It's not too different from Jim adding nickels to Dwight's phone, then suddenly removing them.