StepMoms have a much more challenging path in the blended family adventure than StepDads do.

StepDad is married to the deified BioMom. Because of that StepDad is not a threat to the sanctity and god like status of the worshiped BioMom.

StepMom, is a direct threat to mommy which puts SM at a distinct disadvantage. Even when mommy is the toxic one, directing tons of PAS on the kid to demonize daddy, and package SM as the succubus who violated the failed family by taking daddy, even if the SM had zero presence in the picture at all during the demise of the failed family.  SM is at fault even when SM couldn't possibly be at fault.

StepDads due to their affiliation with Saint BioMom do not generally get targeted in the whole failed family dynamic. Part of that is due to the tendency for courts to marginalize BioDad in the whole process.  Add in that StepDad may sire the next series of issue from the GUBM that may failed family progeny feel much closer to than any additional sibs sired by daddy with a SM and it just gets worse for the SM.  For some reason failed family progeny seem to demonize dad's second family kids far more than they do mom's second family kids.

No idea if this has merit but that seems to be the case.

I'm married to the CP BM in our blended family situation.  As it turned out, SS is an only in our marriage. He is the eldest of 4 all out of wedlock by 3 different bio moms for the BioDad.  I met my DW when SS-32 was 15mos old. We married the week before he turned 2yo.  He asked me to adopt him when he was 22. We made that happen.  My DW and the SpermIdiot were never married. Neither were the SpermIdiot and BM#2 and BM#3.  In our situation the two subsequent BMs are little more than afterthoughts for SS.  SS and SpermIdiot spawn #2 detest the SpermIdiot.  #3 and #4 worship him.

BM #2 and #3 are nothing but afterthoughts since SpermGrandHag bought them out by paying them CS and raising the three younger SpermIdiot spawn while paying the subsequent BMs to maintain control over the three younger half sibs.  SS is the only one raised by the BM.

I agree with points made by Rags.  Also - stepmothers are more likely to be non-custodial than stepfathers, who usually live with the SKIIDs and their bio mother - and it's easier to enforce boundaries with kids when you live with them than when you only see them once a fortnight.  

Also, my own observation is that it stepmothers are a very convenient dumping ground/object to project all the BM's and SKIDs disatisfactions upon.  In 2022 when SD29 sent me an abusive email, she had her say about how she resented ALL the parent figures in her life for not meeting her (extensive) needs.  However, I'm sure she has never told her father about these feelings, and probably not her mother either.  But she can tell me because I'm dispensible, and she would absolutely love to get rid of me. Stepmothers are essentially scapegoats. 

I agree with KES and Rags...

Biology also feeds into the dynamic.  In most nuclear families, the female is the primary caregiver and it's perpetuated in our society that men are the breadwinners and women are the ones that are responsible for home and kids... even when women DO work.. the division of responsibility often follows this kind of pattern.. dad does the lawn, car, home maintenance.. does sports.. mom does most cooking.. most hands on parenting.. dad is discipline and money.  I mean, in nature, the female of the species is hardwired to do this caregiving in most cases where it is biologically needed.  So, our courts and society are hardwired to favor the mother as the primary caregiver.. and to task the father (most often the higher earner) with payment to support the kids.  

The stepmom is in a tight rope situation.. don't overstep into the mother's role.. but, the man often assumes she will replace his EX in that capacity in his home.

Even in cases where the stepmom was not in the picture during the breakup.. the picture can be painted that they are to blame.

And... kids don't want to hate their parents.. so the stepmom is an easy extension.