The best reason that I can think of (correct me if I'm wrong) is that Goodra's stat distributions are all wrong. Look at some of the other pseudo Legendaries.
Hydreigon has just enough Speed to be Scarfed, just enough power to run Life Orb or Choice Specs, and just enough bulk to survive a few hits.
Metagross gets one of the best Mega Evolutions in the game, bringing it back from obscurity. It oustpeeds most other Pokémon, and its stellar Attack stat and ability, plus priority Bullet Punch, make it arguably the best pseudo-Legend once Mega Evolved.
Dragonite has it all; bombastic coverage, decent bulk and the perfect ability in Multiscale, making it seem twice as bulky as it is. Not to mention it makes getting a Weakness Policy boost a piece of cake.
Garchomp has decent bulk, wicked offensive typing and great coverage. While his abilities aren't the best, his stats are distributed in such a way that make him the fastest pseudo (bar Mega Meta), with a crazy Attack stat to back it up.
Tyranitar has insane bulk that is further boosted by Sandstorm. His movepool is deep and his stats are high, except speed. But that's okay! He can afford to go Scarfed, or even Mega Evolve and run a bulky Dragon Dance.
Salamence gets coverage, he gets Intimidate, he gets offences, and he gets the Salamencite. Enough said.
Goodra, however, gets the short end of the stick when it comes to stat distribution and movepool. He's not very fast, nor is he slow enough for Trick Room. He has poor Defence, and while his special bulk is amazing, he just doesn't have the typing to pull it off. His attacking stats aren't very helpful either; 100 and 110 just aren't what they used to be.
Fortunately, Goodra has a rediculous movepool that lets him help himself out a little bit. When paired with an Assault Vest or Leftovers, Goodra can sort of pull himself out of the hole he's in.
So, in conclusion, why do most people not see Goodra as a pseudo-Legendary? Because he's far less useful than the others due to his poor stat distribution.
That, and for some odd reason people hate Goomy :'c