That was done intentionally (of sorts) to avoid the bad publicity experienced from Transformers The Movie with the death of Optimus Prime with all those kids crying in theatres and even one boy in the US locking himself in his room for several months in mourning... erk.. so for that reason they really didn't want to kill of any characters that people would care about.

Yes, it's silly... but it's also silly how people wept over the death of Optimus Prime, whose heroic death was definitely not in vain. Despite Jazz's best efforts - and there's no doubting his incredible courage - he failed to stop Megatron (possibly he knew this and was merely buying the other Autobots & humans time to flee and prepare a better counteroffensive, also to buy more time for Prime to arrive), whereas in TFTM, as Kup said, "Prime did it. He turned the tide." - how cool was it seeing the Decepticons in full retreat with Soundwave carrying Megatron's mortally wounded body back to Astrotrain.

Jazz may have died a hero, but Prime died a martyr... even as a little kid watching Transformers The Movie in the cinema, I understood this and thought that story-wise it was cool (although I must admit that I did want Optimus Prime to return - again, looking at this through a child's eyes). But I never wept over Prime's death and I always thought that Prime dying as a heroic martyr and passing the Matrix to Rodimus Prime was really, really cool (even though Rodimus was disappointing afterwards, but at the time of watching TFTM I didn't know this - there was a lot of potential in the Rodimus Prime character which could've been done really well if he didn't hang on that whole "I'll never be as good as Optimus" thing).

In the G1 DVD interview, Flint Dille justifies his decision to kill Prime in the movie as part of showing that war was consequential and also to remove Prime in a more heroic fashion. Dille does admit that the move was very poorly received by the public and that as far as publicity and selling the movie was concerned, it was a really bad move.

So yeah, Optimus Prime and Bumblebee were strictly off limits lest we have more kids locking themselves in their rooms *sigh* Bringing Jazz really seems like nothing more than a move to appease little kids. :/