IIIRC none of the Prime Exalted Weapons in the game can use their normal counterpart's looks. This has been grinding my gears for years, yes, initially because of Mesa, by now because of Titania whose Prime weapons don't even fold properly under her Razorwing skirt. This also, at the very least, applies to Excalibur Umbra and Ivara Prime, just off the top of my head.
And yes, while most of those frames get Deluxe skins you can pick for the weapon if you buy the warframe's respective bundles, this is still annoying. Regular Mesa's Regulators have so much more thought put into them than Prime's, it's a crying, weeping shame.