Wingardium Leviosa can't be cast on humans
Most levitation spells do not work on humans, as evidenced by the fact that wizards generally need a broom to fly, and Wingardium Leviosa is the best example of this. Wingardium Leviosa would undoubtedly have been the first spell she thought of, but it would not have worked.
As documented in Wonderbook: Book of Spells, which J.K. Rowling had input on:
In response to the catcalls, Hobart tried to move in midair, and
started performing vigorous swimming movements, which produced no
effect. Mistakingly believing that his clothes were making him heavier
and impeding his movement, Hobart stripped thus making him fall ten
feet onto the ground below, as it were the clothes that were holding
him up in the air — they had been charmed by the Levitation Charm, not
Hobart himself. He fell completely naked on the ground, breaking
sixteen bones, and went on the receive a fine for "outrageous
silliness" from the Chief Warlock.
That's why Levicorpus was more than simply a clever jinx: because unlike most levitation spells, it actually worked on humans. It may not have been sufficient to permit true flight, but it was still a step up.
Both Leviosa and Mobilicorpus may not have sufficient lifting power
As the Book of Spells also says:
The Levitation Charm appears to be an improvement on both the Levioso
and Locomotor spells and/or the Hover Charm, being able to lift
objects high in to the air but also allowing one to magically move
them through it as well. It also conveniently defies gravity by
lifting objects heavier than a normal person would otherwise be able
to carry, as seen when Ron used this on a troll's club, something a
first year Hogwarts student would otherwise never be able to do
Despite its strengths, however, the Levitation Charm has one fatal
flaw: it does not work on human beings. Though a human can be
levitated using this charm, it is actually their clothing that is
being affected. The charm is apparently not strong enough to allow a
human to do anything more that float a few feet off the air using this
method, and therefore does not allow the true flight afforded to most
Note that various levitation spells are mentioned here as being deficient in granting true flight.
Though this does not mention Mobilicorpus specifically, we can infer that Mobilicorpus probably cannot lift the subject high enough:
He muttered, “Mobilicorpus.” As though invisible strings were
tied to Snape’s wrists, neck, and knees, he was pulled into a stand
ing position, head still lolling unpleasantly, like a grotesque pup-
pet. He hung a few inches above the ground, his limp feet
dangling. Lupin picked up the Invisibility Cloak and tucked it
safely into his pocket.
Harry Potter and the Prisoner of Azkaban
It seems likely that the same explanation may apply to Wingardium Leviosa.
“If I can just poke the sword through a handle—but how am I going to
get up there?”
The shelf on which the cup reposed was out of reach for
any of them, even Ron, who was tallest.
—Harry Potter and the Deathly Hallows
If the vault had a high ceiling, it is quite possible that it was over 20 feet, and thus that, even if cast on Harry's clothing, the Levitation Charm would not have been sufficient.
It is also possible, from the wording of Wonderbook, that while objects can be moved horizontally by the Levitation Charm, humans cannot be (easily), even when the spell is cast on their clothes. That would certainly be a plausible explanation for what why Hobart couldn't move.
As a side note, one must wonder whether Voldemort truly taught Snape to fly, as he apparently was already making steps in developing a flight spell for humans while he was still in school.