I put it down to release dates.

Devastator and the Dinobots were released at approximately the same time so ended up as "rivals" in S1 and the Movie. Omega was around for S2 so got the job as a more logical combiner counter.

This set the tone for later with the Scramble City bots being staggered. Stunticons and Aerialbots at about the same time and explicitly thematically rivals. Bruticus was on his own in S2 but Defensor fits the rival pattern in the mind of an 8 year old when he showed up in S3. The rest writes itself.