It's weird how they didn't think of that in TFA. "Dammit, man, we're not getting the job done! Where the hell are our absurdly slow-moving dedicated bombers?"
I don't know, those two-dozen Mon Calamari cruisers that were half-again the size of a Star Destroyer would've probably been pretty handy at Hoth, so, probably the same reason as that.
Or maybe they remembered the last time they used absurdly slow-moving dedicated bombers on a planet-killer, they were all shot down before they even made it to the target. "It came from... behind—!"
Or...
That's the thing... with this film, you don't have to go "looking" for problems. You're immediately confronted with them, even if you're not looking.
Oh, goody, it's time for my theory of nitpicking!
Art is subjective and deeply personal, and causes the audience to react in primal, emotional ways that are not readily apparent. A story is a magic trick, a delicate machine made of foundation, misdirection, anticipation, and resolution, whose true mechanics are obscured by surface mechanics that exist for embellishment. Because of that, if something doesn't work, if the emotional core of a story fails to land with an audience member, for whatever reason, they are primed to look to those surface mechanics to explain why it doesn't work (because those are the obvious things, and since the audience member isn't engaging with the story, all they're taking in are the most superficial elements; characterization, subtext, theme, they all aren't connecting), and since they're probably unhappy, they'll lash out whenever they see anything remotely provocative. Thus, nitpicking.
A lot of nitpicking is stupid and self-contradictory. A formative example from my youth would be stumbling on a website explicating all the reasons the movie Independence Day sucked and was dumb. One of the listed nitpicks was that there was a mention of "the Belgian contingent in the Sinai," but Belgium is nowhere near the Sinai, completely missing the use of the word "contingent" or that the scene was between multiple characters from different countries (none of them being the one they were in), at a makeshift international airfield flying dozens of visible flags, so that while they were talking about soldiers from Belgium, there was no reason to think they should be expected to be anywhere near Belgium in that context. That person wasn't enjoying the movie, and was looking for excuses to call it stupid, and saw "English people in Iraq are talking about something Belgium-related like it's next door" and thought "Ah, they're idiots! I'm smarter and could make a better movie than this, because I've read a book."
(Speaking of "reading a book," the newer, hipster version of nitpicking is to superficially analyze the plot based on some story-structure-for-dummies book they read, and talk about the movie is bad because the hero didn't save a cat, or there weren't enough reversals, or some other damn thing, which are usually fundamental-enough structural decisions that they can't influence whether a story "works" nearly as much as what's built around them; for movies where that sort of thing really
is the problem, you don't have to
argue that it's bad, everyone already knows that part. Those are analyses for post-mortems, not trials.)
So, why is it you think slow, plodding space-bombers in a WWII-inflected setting dropping bombs despite the absence of gravity in ESB or Rogue One is fine, but here it "confronts" you with its idiocy? I don't know, while I'd love to strap an eyetracker and an FMRI to a nitpicker who hates a love-it-or-hate-it movie to figure out what moments are
actually ticking them off, no one will let me do it. Since it's pretty early in the movie for it to have burned off your goodwill, and there are only so many things that could've ticked you off (though, admittedly, you could always be retrospectively nitpicking, looking back at the parts of the movie you initially enjoyed colored by how much you disliked the later parts), I'd say it was probably the telephone gag. A mundane, real-world-inflected pratfall in a world you associate with a world of self-serious adventure, where even the old dopey jokes have been polished up with nostalgia to become grand mythic acts, which has been literalized in those little cartoon scenes they've started doing.