Proselytising, or actively seeking converts, has traditionally been discouraged, clearly for several reasons.
I think the most obvious is that Jews who try to convert others draw the ire of those whom do not appreciate their efforts. When you’re in the minority that may not be the best idea in terms of self-preservation.
At several times throughout history Jews have been banned from doing it by others (insecurity?) or by Jewish leaders (fear). Converts are easily suspect of being mutable or insincere in their motives. Even though it is explicitly stated to love the convert, it rubs many the wrong way, mainly those who believe in their genetic superiority. Those that deny the fact that intermarriage has always been.
If you believe that Jews are inherently a people based on shared genetics, then converts are as adoptive children. Do you then welcome their children, grandchildren, as ‘born’ Jews? Where do you draw the line? Technically they are ‘born’ Jews, but there’s many who wouldn’t feel that way.
The other obvious reason for the need for converts I think goes back to the original convert, Ruth. The fact is that a small population cannot sustain inbreeding for too many generations, human genetics needs variety to thrive.
Then there’s the whole numbers game. The more of us there are, the stronger we will be. That’s just survival.