Iran got an advanced centrifuge design from A.Q. Khan that is extremely difficult to operate in practice. (We also interdicted the supply of some of the advanced machine components it requires.) For some reason, they stuck with it, and eventually got it to work. That's why it's taken them so long to get significant enrichment.
Now, if they were on a crash program to build a bomb, they could have abandoned it and pursued a simpler earlier Soviet design. So I agree it's not their first priority. Indeed, the fact that they stuck with the expensive but efficient technology suggests that they want more than to just "build a bomb".
However, that doesn't preclude a dual program, to pursue civilian uses, but retain the option of building a bomb as well. I suspect that's their real intention, and there is evidence that they've done some preliminary weapons work (such as implosion devices and delivery systems, IIRC). Iraq actually used (chemical) WMDs against them in the 1980s, and with all the U.S. activity in the region, they may eventually want a deterrent.