You can read the answer right off the study: no, lack of intelligence is not the main reason because the correlation coefficient between IQ and various measures of religiosity are not that large (e.g. -0.25 for fundamentalism). It is an important reason, as is education (as the study explains, the correlation persists when level of education is accounted for). A combination of extreme education and IQ does seem to be enough to virtually eliminate strong religious belief. (This is a broadly supported conclusion, though causation is not determined.) The inverse correlation with IQ holds, at least roughly, with belief in the paranormal also--the effect is frequently seen but is not so strong as to appear to be the only important factor.
The other factors are hard to pin down. Some studies indicate that religiosity of children (once adult) is not strongly correlated to that of their parents; others find cases where the transmission is more reliable. Religious beliefs of friends and spouse are strongly correlated (see first link), but it is unclear to what extent this is cause vs. a consequence of having the beliefs. There is very likely some strong societal or parental component given that, for example, 2/3 of the residents of Utah identify as Mormon, and the "natural increase" of the population is considerably larger than the "net migration" (as found in a table of Utah demographics linked here).