why did they change Galactus into a cloud?

I think it was lack of imagination as much as resources. It takes time and effort to make these things work, and they didn't really want to.

Which means a different creative team should have been in charge in the first place. You need people that do want to make it work.

So what exactly happened to this spin-off film?

A couple of things.

First off Tim Story is, to be kind and brief, not an imaginative director.

Second, in the "Marvel Ultimate Universe" Galactus made an appearance as a interstellar cloud of planet stripping hive mind robot drones called Ga-Lac-Tus. The overall story was handled very well with the FF, Nick Fury, Marvel's Captain Mar-Vell and others appearing. It was hands down a better portrayal of Galactus than FF:RotSS.

It is my opinion that the people in back of the FF "franchise" (as this is a third version by the same studio, not counting animation and the Roger Corman unreleased film) do not know what to do with the rich history of this group. Some of their powers are either, on the surface only, silly (a man can stretch - look at how they've been portrayed by DC) and a person who turns invisible having been done over & over (while not attempting to properly utilize her power of force field generation). The Avengers Universe films show good writing, direction, casting and a respect for the genre can be rewarded with great films that give the audience action, plot AND good to great character development.

"It's just television, get over it!" - David Letterman

I think it can work, but I can understand believing otherwise. In the director's mind, something cosmic and infinite like Galactus was more than just a man. It's kind of the same reason you rarely see God as a bearded old man and is more often either formless or of many forms. I realize the current comics have said that how Galactus appears isn't his true form and just how he presents himself to humans.

Yeah, movies that feature giant robotic humanoids never succeed at the Box Office.........

That's a totally idiotic comparison. The Sentinels on DOFP were still ROBOTS, even if they didn't look exactly like the comic book counterparts. Your comparison would only work if they also changed the Sentinels into a cloud