I'm a stepdad. I started reading, and occasionally posting, here when I first got the gig seven years ago.
It became clear quickly that we simply have it easier. A stepmum does a lot of hands-on parenting, and there's a war about if she's overstepping or should be doing more. I do the same, and I'm (completely unfairly) almost revered for being so committed.
And yes, we tend to be with the stepkids more, since the mum often has them more. We had our own little baby four years, and because the stepkids are only away every other weekend, I know them just as well. They're as big a part of my life. So it's easier to hold them as 'true' family. Plus we don't carry a kid around for nine months, nurturing it with our own body, so the difference between a stepkid and our biological kid doesn't feel as stark, I guess.
I don't get half the problems with the dad as many stepmums seem to with the mum either. I mean, I can understand why my wife left him and think he's lucky she chose kicking him out over murder sometimes, but we muck on ok with little conflict. I think he's happy someone has taken some of his responsibility rather than jealous?
Possibly single dads often look for a mothering figure while single mums are more likely content with someone who will understand that the kids come first but not have as many parenting expectations?
There's other examples, but the short answer is we just have it easier in general, I think.