It's what happens when you traumatize a legion of children who, whether they grow up or not, become adults and dominate the discourse.
But more seriously there's the branding element of it as well; for better or worse they somehow didn't know what they had in Optimus and the rest of the brand at the time (to be fair, it was early days for the modern toy IP/brand framework), and Hot Rod and the New Generation was an unsuccessful attempt at reinvention at the wrong time. From then the corrective steps were to restore the status quo (or at least the Optimus Prime of it all), reinforce early-year presence in branding (84-85 characters would be in Pretenders and Action Masters, and led G2), and find ways to keep trademarked names active. But by the time all were in place, Transformers was in need of a refresh as a whole.
(I think the presence of the movie in the public consciousness is also a bit overstated there. At most casuals will recall they killed Optimus Prime, and also know they brought him back after. Many may not even recall Megatron becoming Galvatron; I've seen more cases of the next memory after OP being Unicron. It's more the fandom and franchise, and even then the trauma/distaste for that plot point and the need to restore and maintain the brand icon took precedence over novelty/change, ironically.)