Polygamy is sensible in a tribal culture where fertile females are limited and males are far fewer (usually due to warfare). Survival of the tribe is in doubt, so the best is made with what little there is.
Robust cultures find a 1:1 male/female pairing is optimal for raising the next generation. Living in the resulting luxury, new generations opt for novelty, forgetting the hard won lessons of the past.
Well, practically speaking, fertile females are always limited - i.e., there are never enough of them (from the point of view of the tribe's survival). More precisely: The number of fertile females is the so-called limiting factor. Kill off 90% of the males overnight - it doesn't matter: Theoretically, the same number of females could be happily pregnant next year.
By the same token, from the point of view of the males, there are always too many males (i.e., too many co-competitors for the attention and affection of the females).
Of course, as you correctly point out, from the point of view of the tribe, a 1:1 ratio is optimal.
Regards,