(Wanna take this one, Twilight? No? Okay.)
Short answer: Twilight has a knife at my throat. GERKKK
Long answer: Humanoid Flash Sentry was an average character, written in as a love interest. They glomped on each other, they danced and made blushy faces. Flash Sentry as a pony was basically non-descript. He didn’t feel like a parallel unlike the rest of the humanoid Main Six. Just some guard with a Twilight magnet.
I didn’t appreciate that. I needed to make the canonical characters work. So I came up with my own theories and characterisations and put them in art, similar to how I have Shining Armor/Cadance and Shining Armor/Twilight.
Apparently I’m changing the opinions of some people about Flash, even if they are non-canonical. And if they do expand the Flash character to the stallion I had in mind, or not, I will always have my version and what it has become.
Plus I make them really cute together.