Well the reason is fairly simple, it's not like you can make the jump and actually be good at dota without at least I don't know, 2 years of practicing hard. And even then who knows if you'll be competitive.
Plus you have to consider a lot(all?) of the pro players in lol have salaries. They get paid, winning or not. In dota, only a few teams have salaries, most of the earning are from tournaments, which means if you don't win, you don't get shit. Meanwhile you can be a NA LoL team that has 0 chance of ever winning an international event but still get paid every month. Not that there's a lot of international events in LoL anyway, and I guess that's another minor thing. If you're a western lol pro, you mostly play in your region, which means it's easier comparitively to be top tier. It shows heavily when they do worlds and most western teams get decimated by koreans and chinese teams like fucking tier2 trash. In dota, being top NA or top EU doesn't mean nearly as much when so many tournaments have basically all the top tier teams from every region. If you're not at the level of the chinese, or cis, or EU, or NA, whichever is strongest at the moment, you're still not winning much.