Because the animators didn’t feel like making you a swimsuit design.
 
Plain and simple.

Headcanon: Bon Bon intended to go on the cruise with Lyra, but she became sick and thus was bed-ridden. Lyra originally wanted to stay and take care of her, but Bonny insisted that she go on the cruise by herself as she had been waiting so long to go on the trip. Reluctantly, Lyra agreed to go, but made sure to keep in contact with Bon Bon and send her pictures and videos of the cruise, along with Derpy, Vinyl and Octavia wishing her well to get better.

That whole witness protection thing, you know, “Bon Bon”?

Well good question. Im sure she is seasick like Applejack, but Applejack is a maincharacter and Bon bon just a background character.

Because, Bon Bon… don’t you remember what happened “the last time”:derpibooru.org/1323743 you two were on a cruise together?
 
Lyra has a serious problem.

Ouch

Because you were on the secret misson of buying Lyra an engagement ring.

Lyra made a lot of “favours” to the captain.  
>tfw Shino doesn’t make part 3

Grounded. Not allowed to go.  
Shouldn’t have mouthed off to your mom.

Because you were on an important mission to stop a supervillain.

Those CIA internships are truly unforgiving.

Because a short circuit caused the tape to self-destruct prematurely.

But…maybe that image was being shown from the POV of Bonny, admiring her girlfriend (note Lyra’s happy little smile)?
 
:)