It's not so much the content of the movie giving it a poor performance at the box-office... it's about the marketing. This is why most big movies spend more on marketing now than the actual movie (Bumblebee might have had a budget of 130million, but I read somewhere that it has to make 250-300million to make money... probably thanks to those global promo tours they had in most countries... except ours).
For the actual marketing of the Movie, most of the trailers and TV ads I've seen focussed on the girl and the relationship between her and the car.... and I only saw one TV trailer that focussed on the robot fighting and special effects. If you spend a lot of money on certain elements of a movie and then don't promote it in your advertising, people aren't going to go see it, because they don't think there is any. We know its there because we are dedicated fans who have been following the production of the movie... but regular people who made up 99% of the billion dollar average of the first five movies, will think it is a drama, or more like ET than Transformers 1-5. It doesn't even market itself as "Transformers", with that word being absent on movie posters and advertising... so it isn't even cashing in on that multi-billion dollar brand name. Just calling it Bumblebee doesn't get the same word-association that a Transformers-Bumblebee would.

And my third point - Bumblebee tried to market itself as a family movie, which is not where most movie money comes from. Only three of the top 20 highest grossing movies are PG or G (in Australia)... there is a reason why 85% of the highest earning movies are M rated, because the age demographics that are more likely to go to movies prefer to see something that isn't "kiddy friendly". Paramount in Australia had the option to have the movie rated M, but fought to have it PG... and it wouldn't have stopped parents taking their kids to it (most people I know still had their kids see the earlier ones), as they would have watched it and seen that it was kid-friendly.

And my final point, is that if you have a greater proportion of your audience being young kids, it means less money being generated (after costs), as kids tickets are cheaper than adult tickets.
A great movie doesn't guarantee a great return... and a great return doesn't guarantee that it is a great movie - as Transformers fans we know how much that is true, when comparing this one to the previous five.
And sometimes, there is just no obvious or rational reason why a movie succeeds or fails. Just look at Avatar - I still don't get why that movie earned so much - us2.7billion... for a movie that wasn't that exciting or spectacular, and to me was very predictable. I think it was a good movie, but not great... certainly not great enough to have earned 30% more than the next biggest movie... and that only 7 movies earned above 50% of its total earnings.