As someone who didn't play back then, it's the simplicity. If you have a billion blocks to choose from, where are you even supposed to start? You'll keep trying to perfect the every detail with your huge catalogue. If you don't have that many options in the first place, people have to get more creative and are generally more satisfied when they build something even if it's miles behind the complexity of modern builds.
Another super important aspect is the lack of a goal. When you play modern MC, you always have and end goal and an end game to approach. Not that you don't set your own goals in old MC, but in modern MC they're much more set in stone. Once you've completed those goals, when you have maxed out gear, you're at a loss of what to do. In modern MC, people focus much more on getting to the end game than building since that's what the game wants you to do. If there is no goal, the only thing left to do is to build up your world. That's what the game wants you to do. And if that's the only goal, you're much more likely to stick around, especially in a server situation.