there is rarely a new manager

why is it so rare now for a new manager or rookie manager to join the league?

Hi, 
I've said it in the past and I'll say it again!
When joining a new league, we should automatically start in the rookie tier and have to work our way back up to elite.
You might say it's unfair to new players!
But not only will they earn more experience points to level up faster, they will also see in the research how you're building the car and they will be able to replicate it, the next season and be more competitive. After all the way you build the car matters a lot. 

100% agree with this, another thing is you should have to be a certain level to host a league, the amount of leagues with hardly any people because the host has never played the game is ridiculous. In my opinion you should have to be at least level 15 to make a league, but that’s just me.

That sounds fair enough, level 15 a new player I guess would reach in 2 and half seasons. Depending on the level of competition and number of races. Guess it could be something they could look at for public leagues and allow private leagues at any level.

I have to disagree with this, I get what you mean but it doesn't work in all situations. First of all, it's frustrating to work your way up from Rookie every time you switch leagues, particularly in leagues with long seasons. 

As well, if you take a league such as Liga de Leyendas which only has an elite tier, there would be no way for new managers to join as they would not be able to be promoted from rookie. Unless you use bot accounts, but that defeats the point of giving rookies more experience against strong managers.