Just like Marvel, DC, Star Wars, Star Trek, Dr. Who, and every other major sci-fi/fantasy/horror series/franchise out there.
Honestly I think even if IDW just used characters from the original toyline from 84-89 there were over 500 characters in that toyline and then you throw in characters from later toylines (like Beast Wars, Prime, Cyberverse, and etc) brand new characters (like Drift, Windblade, and Victorian), comic original characters (like Rung, Impactor, and Rack n Ruin), and the Japanese characters (like Overlord, Star Saber, Deathsarus, and God Ginrai) then you have more than characters and not to mention more worlds than just Earth and Cybertron, (you got classic worlds like Nebulos, Junk, Goo, and the more new TF colony worlds) so there is more than enough for 4-12 ongoing titles.
IMHO two Transformers series set in the same universe just isn't enough and when they split the series into 2 different titles that was actually when it got great. MTMTE/Lost Light was one the GREATEST comics published in the last 20 years because it took the Transformers into new and different directions and showed that the Transformers could be more than just giant alien robots beating the crap out of each other for 20 pages.