It's because that's the first impression the word stepfather gives. And most people is afraid of asking about the details so the assume to let you give the explanations, it happens to be also a comfort issue, to not waste so much words into a thing they believe it bothers you more than them. With regard to that, you have to accept that.
If don't want to live giving explanations just say my father died, cut sharply changing the topic of conversation and they'll understand in a minutes.
A good reason for them to label a stepfather on such a demeaning way is because most people think marriage is sacred and concubinage or remarriage is the worst thing along with killing.
The case of your stepfather is not a sin, is perfectly blameless as Lord Jesus stated that.
Of course many women widowed prefer to keep with the remembrances of their beloved husbands and never think on marrying again, other times are their children who ask them expressly not to do it, but this is only decision of the widow and by "the request of their children" but it's not bad to marry again after having a deceased spouse.
Thinking on this last thing, If you are so annoyed to explain this all the time after you say: I have a stepfather and they reply that he doesn't even deserves the word father behind step, you should tell them sharply that it's not a sin that a widow remarriages.