Things like Monferno and Gabite simply don't have the raw stats needed to compete.
Raw stats aren't everything ... Gabite has a base stat total of 410, and Monferno = 405.
Both higher than Ariados who has BST of 390.
Yet in the past I've used said spider to take out half a OU team with a little luck, some prediction and super effective attacks: Gengar (BST 500), Azelf (BST 580) and Swampert (BST 535)
Admittedly given Ariados lower base stats it would be difficult to accomplish this on a regular basis, but still this little example hopefully demonstrates that base stats aren't always the be all and end all.
Just something to think about ...