I suspect that evolutionary explanations of masochism suck for the same reasons that evolutionary explanations of homosexuality suck: these sexual orientations are the product of developmental factors, rather than being evolved adaptations. Purely social/environmental explanations are also pretty bad.

Homosexuality is related to certain developmental factors, particularly prenatal hormones. There's some preliminary evidence that masochists are more likely to be queer: BDSM practioners disproportionately report non-heterosexual orientations relative to the general population (one example: this study found rates of 51.9% and 20.6% for female and male bisexuality, which is way above the general population, though this sample wasn't random). If homosexuality is related to developmental influences, and masochism is correlated with queerness, then I would hypothesize that masochism is related to developmental influences, also.