Hmmm... I guess there could be a few reasons.
People are more likely to post about issues than things going great.
Also, maybe the in-laws relationship tends to be a bit more complex than a kind of... direct relationship with your own parents? It's you, OH, and in-laws and OHs can feel stuck in the middle if their parents and wife/husband/partner are having issues... so I guess it's reasonable to ask for outside viewpoints if OH isn't wanting to get too involved or disagrees with you regarding the PILs? Though I guess it's uncommon to see a FIL post. 
For stepmums, and ILs actually, I think anything where different families are joining together, can naturally have issues, big or small. Sometimes we don't realise something that's normal to our family isn't normal for other families until we're a bit mushed together!
I have seen a fair few negative posts about DILs. Normally complaining about their choices for the GC. Sometimes threads here can get quite vicious both ways!