Main reason being, the costumes would look really ridiculous on-screen.
If you ever see the Marvel island of Adventure theme park, it's apparent.

They got a few things wrong, but got most right.

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With Transformers, though?

One of the main gripes was Bay's thorough dismissal of the establish design aesthetic of TFs in favor of him being able to include whatever new flashy GM (read:American) cars he got a boner off of.

He also wanted them to be over complex and ugly, to symbolize their alien nature... ignoring established canon that the robots were initially manufactured as service and military machines, but later gained self-awareness. They shouldn't be ugly, but presentable.
If anything, they should look as refined as a car would, with the mechanisms hidden for the most part.

Other shit?

• Instead of accidentally crash landing their ship on Earth and remaining dormant for millions of years, the new movie has them crash land in 2007 as self-propelled metallic turds that can reshape themselves into any form they please.

• Instead of the Decepticons seeking to deplete the planet's energy sources to return to Cybertron and rule, they're instead seeking a mystical cube that can make a Mountain Dew vending machine and a cellphone turn into robots so they can take over Earth.

• Instead of the being the runt of the litter, Bumblebee is now larger than Jazz and a muscle car... but can not talk, and instead plays songs to convey his thoughts.

• Spike and Sparkplug are now called Sam and Ron, man.

• There is no Soundwave, but theres a little... thing that turns into a stereo.

• Why does Optimus Prime have flames? It betrays his character.

• Jazz dies? You just introduced him! WTF?! (Always gotta be the Black guy, right Hollywood?)

• The film spends more time with the humans, where in the REAL TF movie, the only organic characters were Spike and his son (and a fucking fish).

• The script ends with the plot device gone and the main villain disposed of. Now there is no reason for the TFs to be on Earth. How you gonna make a series of films out of that?

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IMO, they botched the handling of the actual film (aside from the CGI).
Even worse is, the rift this has caused in the TF fandom, which is as die hard as Star Wars/Star Trek, only on a smaller scale.

And with Hasbro basically giving the middle finger to the incarnation of TF that people know best? I'm pretty much through.

I'm gonna cop the figure in my avy (Hot Rod) and the Masterpiece Starscream and be done with my collecting.

I got better things to do tonight than cry.