I can see some reasons why Pinkie Pie (formerly Surprise) lost her wings, but I don't think it's "it goes against her personality".
G4 clearly has a pattern going on where they only want two of each pony type (prior to Twilight's ascension) in the Mane 6 (versus G3's Core 7 having Sweetie Belle and Starsong as the token unicorn and pegasus respectively). Fluttershy was an earth pony, Posey, in the original G1-reincarnation plans and is probably the most drastically changed when they were renamed, given that Posey is a gardening pony. They started redeveloping her based on the butterfly theming of G3 Fluttershy while keeping Posey's colors.
Even before establishing in lore that she's a "fish out of water" among pegasi for sticking close to the ground, giving her wings to make her seem more butterfly-like or giving her flight so they can put her in higher places (such as trees with bird nests) in the animation meant someone's wings had to be forked over... and it sure wasn't going to be sporty-speedster Firefly/RD's.
Fun fact: her G3 counterpart, in an era where cutie marks weren't 100% connected to talents and personality, was a photographer, which explains why her name looks like a copyright-dodging spoonerism of "Shutterfly".
Another plausible reason for ditching pegasus Pinkie (after she stopped being Surprise) is that, because of G3 Pinkie's prominence, they didn't want to mess with her design too much. G3 Pinkie Pie was practically the face of the generation.
I grew up with G3/G3.5 and, while I had a whole bin of ponies and Core 7 stickers all over my bedroom door, back then, at about kindergarten age, I could not tell you the names of any of them except for Pinkie Pie and Rainbow Dash, the latter because I had a plush with her name on the tag. Pinkie Pie was on my birthday cake. The first pony I remember actually being able to read the name of on the pack was an Easter Bunny Pinkie Pie. The big G3 DS game (that I never owned but longed for) was Pinkie Pie's Party. If you asked anyone who knew the bare minimum about MLP at the time, they could at least tell you one of them was Pinkie Pie.
Toy companies like putting the pink one in the forefront. It's the same reason why less-invested people are likely to be able to at least name Draculaura if asked about Monster High.
With how big Pinkie was in G3, while I personally don't think adding wings would impact recognition, Hasbro is likely very protective of their pink moneymaking brand figure. This explains why, design-wise, her cutie mark is a straight redraw (down to the number and color of balloons) and her mane and coat colors are the reverse of G3.
Before the argument about G4 Pinkie growing up on a rock farm being devalued if she were a pegasus crops up, consider that this bit of lore was likely written when it was firmly established she would be an earth pony. Key parts of the Mane 6's backstories in G4 as we know it hinge on their pony type and, considering Surprise and Posey switched types when they became Pinkie and Fluttershy (the latter even changing talents), their lore either changed or wasn't fully drafted until everything was cemented.
Moving on to the point of Rainbow Dash being changed, I think, in rebranding Firefly, they landed on repurposing G3's Rainbow Dash because of the "Dash" in her name (suggesting speed, good for the sporty character) and to give her a popularity kickstart by tying her to one of the more popular G3 ponies (after Pinkie Pie obviously).
However, there was a space for her original G3 personality in the cast, taking Rarity's role. While there seems to be an emphasis on wanting a unicorn in this role given Rarity was originally going to be either Sparkler or Glory (both were inspirations), they were able to change Rainbow Dash to a pegasus without issue, so making her a unicorn would be just as easy. Why wouldn't kids want a rainbow-hair unicorn? G4 Rarity even inherits her accent from G3 Rainbow Dash.
As for why this role would need to be a unicorn and couldn't use earth pony RD, Rarity's role places additional emphasis on design and creation while RD focuses more on styling. While RD does have the potential to expand, using a unicorn with levitation spells would save the animators from having to figure out hoof or muzzle physics for certain parts of the process (hand sewing, using scissors, etc.).
The fact that we have Rarity as we know her instead of Unicorn Rainbow Dash is likely due to wanting to hold onto the original concepts as much as possible by giving her Glory's color palette and trying to incorporate Sweetie Belle (who was also a unicorn in G3) as a "matching" little sister character. Also, being named after the G3 Princess of Unicornia... no wonder she's spoiled.