There were three forces pushing the early Saints in the direction of polygamy.

1.  Mormonism has a much weaker view of historical progress than mainstream Christianity.  They are comfortable with viewing the OT folks as pretty undeveloped and primitive in their notion.  But Mormonism is emphatically Restoration Christianity.  Our view has always been that the ancients knew Christ and the doctrines of salvation and many plain and precious truths that have been lost.  Mainstream Christianity could easily dismiss the OT fathers polygamy as a relic of barbarism.  But early Mormons had to take those gentry’s polygamy more seriously than that.
2. There was a practical-theological problem.  More women converted than men.  That would be a practical problem whatever the theology.  But a theology emphasizing marriage as the ultimate goal makes the different numbers of men and women a theological problem also.  (I’m not saying that eternal marriage was taught before polygamy.  Just that having the one, along with the disparate numbers, creates a need for the other).
3. Much more speculative here:  Testosterone levels seem to have been in decline among moderns for quite some time.  There is not a perfect correlation, but higher T levels seems to roughly equate with higher sex drive.  Further, the men among the early Saints would not have been circumcised, which also roughly correlates to a higher higher sex drive.  There are countervailing factors, of course–poor nutrition and physical exhaustion.  But whatever the levels of interest in sex, mainstream beliefs in that era that sex was for procreation only and that women did not or at least should not enjoy sex would make polygamy more attractive to a set of lusty fellows, and their contemporary apologetics for polygamy emphasize these kinds of points to an extent that is embarrassing for us their descendants.