I'm assuming you mean all three of steel's weaknesses, and I would actually like to suggest a couple of reasons for these weaknesses being common. Steel has so many resistances, including the unshared dragon resist, making it arguably the best defensive typing. Therefore, it is to be expected that steel-types will be used frequently in OU, and that's without considering the offensive powers of the likes of Metagross and Scizor. People will want to get past such common steel-types any way they can, and the best way seems to be by hitting them super effectively.
Furthermore, two of the types it is weak to are also the only types that hit others for super effective damage. Fighting is the only thing super effective vs. normal, and the same for ground vs. electric. Furthermore, both fighting and ground hit the most types for super effective damage, with fire only just trailing behind.
Yes, that's pretty much the reason they gave those weaknesses to the Steel type; they had to balance its defensive abilities. I still feel they didn't make it 100% right exactly because of the Dragon resist. It should have been given instead to some other typing with less resistances but also less common weaknesses, so people wouldn't always resort to Steels as their ultimate defensive typing. Giving Dragon resistance to Steels only didn't help much in stopping Dragons from dominating as they can easily use Earthquake/Fire Blast or even Magnezone to stop them. And then proceed to spam Outrage/DM and kill everything else. Even being an Altaria.