The Show didn't elaborate it much and also show is not finished yet but source material did elaborate a bit:
Below is the graphic image of one kid's birth with the text "...the ones who survived" (suggesting some died too):
Also in from another panel:
This shows that Hargreaves tried to adopt as many as he can but he got only seven.
So there is a ...
Most likely because Leonard had to be alive long enough to instigate Vanya's wrath. The series of events that made Vanya literally explode was:
Vanya meets Leonard
Vanya falls in love with Leonard
Leonard gains Vanya's trust
Allison attempts to split Vanya and Leonard.
Vanya slits Allison's throat.
Leonard and Vanya run away and he tries to convince her to ...
Well, we don't necessarily know it's God (with a big g) it's implied to be God, however. Though, the interesting part of the scene is not the depiction of God in my opinion, but rather, the character doesn't recognize Klaus and it's indifference to him. Regardless, I digress,
This answer depends entirely on how much you want to take from the comics or not.
If you look at the FIRSt apocalypse that has happened, Leonard was still alive: we know that because of Number Five, who found Leonard's fake eye in the rubble.
So we can guess that they wanted to repeat it again, which required Leonard to live up to the end of the world - he must have been somehow the catalyst for the apocalypse, while in the second one, ...