although the academy is open, you should know that there are thousands of coalitions, .. there in the game with the academy open, and if the new player joins a coalition random, the probability to join yours is minimum.
It takes time for a coal to grow. As there are thousands of start up coalitions, players usually press' join random coal' and they end up randomly in one :)
If you are referring to the frequency of players getting pushed into your coalition through the "join a random coalition" challenge by Leonidas, it is attributed to two things: The number of coals in the game and like you stated, less people are joining the game. More so the latter like you suspect. :o