Because sadly, Flash doesn't have any personality, he's just there for Twilight to fall in love with and there is no "spark" between these characters, i mean she bumped into him, then she did that again, she ruined his shit *boom* Love..It's just not fun.
If anything his pony counter part has more character. He's part of Cadance's royal guard. He seems to be her announcer and all he's given twilight Is a kind smile once or twice. I do agree that brad sentry is just their for the sake of being a love interest.
that doesn't mean you have to hate him sure he's just there for Twilight to fall in love with in the show and in the show he has no personality whatsoever, but try thinking of Flash as a hidden fill in the blank puzzle if you will, maybe just maybe you as mlp's loyal fandom are given the oppertunity to pick up the pencil, open the Mad-libs book, and write in an adjective in the space provided. Have you ever thought of that?