>>41321574A combination of stats and niche use.
Bug/Steel is an amazing typing. Only weak to 1 type (Fire) which is somewhat rare to come across and many key resistances. The problem with some of these shitmons is that they don't excel at any niche, and are hampered by their stats.
Back then, Scizor was a powerhouse with amazing priority, and Forretress has a pwoerful niche as a rapid spinner and hazard setter before Ferro came in and cucked him. Genesect is self explanatory being a Legendary so the BST is high.
Wormadam has fuck awful stats, Escavalier is too slow and doesn't excel at much (If you wanted a similar typing and to hit hard you'd just use Scizor instead), and Durant lacks a lot of moves to make use of, the fact some sets run dig is pretty laughable and shows how myopic it is to use.