1. Kids have a ton more time.
2. Ttrpgs compete with video games for time.
3. Alot of the newer rpg players are more casual which means they aren’t going to want invest the time to try a bunch of different systems.
I think there were a couple factors that contributed to this in the early days:
1. D&D, while still the big dog, was not quite as huge. That meant more people were likely to hit RPGing through other games in the first place.
2. The vast majority of early RPGers were SF fans (as in, active fans) or wargamers. And such people were more likely to be interested in things beyond the experience D&D offered early on and actively look around.
Real game changer was advent of flat rate broadband internet. With that came streaming, void, social media, profileration of online multyplayer games.
TTRPGs are part of entertainment industry. They compete with all other forms of entertainment for same primary resource - consumers free time.
Back in pre internet and web 1.0 days, there were lot less options to kill free time. If you wanted to see movie, you could go to cinema, rent it, or wait till it came on tv. Tv shows had set air slots and you either watched it or recorded it. Now, you have dozens of VoD platforms, you can binge whole seasons of tv shows with couple of clicks on tv remote. Same with music. What you had is what you listened and then alternative was radio. There were very few games worth spending dozens of hours, and spending 1000+ hours on single game was almost unheard of. Now, if you play online game for 2-3 hours a day, in a year you have close to 1000 hours clocked on that game.
With limited other choices, playing multiple games was not that uncommon. Now, we have abundance of other options for leasure time and for lot's of people, TTRPGs are just one of them.
Also, there was less casual players. It was more of a niche hobby. People who played were more invested into TTRPGs as a whole. Let's call them "hardcore" gamers. People like that are more interested to explore other things inside hobby.