Painting with a wide brush here.
Shoulder launched SAM's were desinged to fill in the gap between the small arms envelope and the vehicle or base mounted SAM's envelope. While traditional AAA (20+mm guns spewing a million rounds a second into the air) are pretty cool, lack the kind of mobility needed in a fast paced battlefield environment. AAA also has quite the supply train. Things like SA-7's and Stingers are highly mobile, require almost no maitenance, and cost less than a few minutes worth of AAA fire, not to mention the training aspect.
As far as Irans SA-7's being better than anyone elses, quite possible. No matter where its made, an SA-7 is still an SA-7, the difference is in the bells and whistles.