Several months ago I requested I Became A Dog 2 be added to sr.c and it was accepted. But a few days ago I requested that the 3rd installment of the series be added as well, but it was rejected because "We are not accepting short/trivial games unless otherwise notable." So my question isn't really why was it rejected, rather why was it rejected when the 2nd part was accepted.
You just said "But a few days ago I requested that the 3rd installment of the series be added as well, but it was rejected because "We are not accepting short/trivial games unless otherwise notable."". There's your answer. Just because your first game request gets accepted doesn't guarantee your second game request to get accepted.
It looks like that the first game your posted barely gained any traction as it only has 3 runners so it's unlikely that the next instalment of the game would get accepted. It may get accepted if more runners come into the game but right now, it won't.