Just look at how many of their fans show up at away games. I bet well over 1/3 of Candlestick Park was wearing black and waving Terrible Towels. Whenever Heath Miller caught a pass, the "Heeeeeeeaaaath" chants were as loud as anywhere.
And, I haven't been in sports stores all over the nation, but the ones I have been in always have a large Steelers selection. Around here (Atlanta), there is way more Steelers stuff than Cowboys.