I blame whoever thought that mattered.
Media companies have gotten too cute with their properties. Every major fictional character these days needs a "[INSERT NAME HERE] Day" that basically amounts to a big social media push every year. Hasbro wanted in on that, made "Transformers Day," and decided that it would be the anniversary of the G1 cartoon debut.
And once you've sniffed your own farts like that, the idea of releasing a new movie on that oh so sacred day seems like a really fun idea, even if it's not a great day for the sort of movie you're releasing.
Still, I can't help but think that even the bad release date could have been overcome had there been interest.
The biggest shame of TFO is that while it is a fun movie for Transformers fans, it's a great movie for general audiences. My husband loved it, and he is far less into this stuff than I am. I had a good time, but I could clock where things were going like ten minutes in. He, on the other hand, was genuinely shocked by the "twist"(?) that D-16 was Megatron.
So they made a movie that could absolutely appeal to general audiences with a good story, but general audiences just don't care. They don't see an all animated Transformers movie as anything to really get excited for. And while bad marketing and a bad release date didn't help... I can't shake the idea that Hasbro and Paramount may have bought into the brand's hype a bit too much? It can be good. Really good, and TFO proved that.
It's just really, really hard to get general audiences invested in Optimus and Megatron's personal stories like they are with Luke and Darth Vader's, for example.