Thank you for this!
And the scene is so adorable. Their swimsuits really reflect their characters. Rarity probably bought Sweetie's. Applebloom's is farm-functional, and Scoots, ha, is waaay too outgoing/unshy to wear a swimsuit when she doesn't absolutely have to; she just doesn't need one. And it really makes the whole thing cherubic. She doesn't care, why should she? It's all friends here.
Cute and good Picture. ^^
And that water looks so... real... I think... I prefer to hide in the woods... Don't like open areas. XD
D'aww! Sweetie Belle looks so cute with a swimsuit!
Plus, Scootaloo is NAKED!!!
Swimsuits are to girly. Scoots is a tomboy and way to cool to be seen in something like that. Since she is not really naked as this is a natural state of all ponies and clothing is treated as an accessory more then a means of modesty. She as well as the others do not even bat an eye at her.
That or she is to poor to buy one. Which would be a mute point as she still probably would rather not wear one. Especially one like Sweeties.
So there, that's what I think is the reason. Scoots is just being natural. Naturally Cute! They are so adorable.