Why are fast offensive Pokemon, like Spectrier and Iron Bundle, often more broken in singles than doubles?
Why did the council decide to suspect Basculegion instead of last respects?
Those fast offensive Pokemon typically have pretty low defensive stats. In a format where each player has two Pokemon in play, these glass cannon offensive Pokemon tend to take one knockout and then get swiftly knocked out in return. When this is the best case scenario, these Pokemon don't tend to see a ton of use. These Pokemon can also suffer against speed control; moves like Tailwind, Trick Room, and Icy Wind can expose these Pokemon's low defenses by preventing them from moving first. Protect makes a big difference too - a lot of glass cannons have strong single-target attacks, but don't have strong options for spread damage, and firing a strong single-target move into a Protect puts them in danger.
Deoxys-Attack is legal in BW, XY, and SM DOU, and generally suffers from this problem just as much as things like Spectrier and Iron Valiant. Iron Bundle can carve out a niche for itself as a fast Icy Wind user and generally sees more play than other fast frail Pokemon as a result, and Flutter Mane doesn't really qualify as "frail" with base 135 Special Defense.
Basculegion was suspected because Last Respects isn't broken on every single thing that gets it in DOU. Houndstone is bad, even with Last Respects, and Basc-F isn't awful but isn't broken enough to do anything about (though some may disagree), so Basc-M got the axe rather than Last Respects. While I can't speak for the council, I imagine if Basc-F were suspected tested, Last Respects might be considered instead (and Houndstone can just suck even worse idk).