Why can't my husbands ex wife except me as his new wife+stepmom to her child and stop making trouble for me

tom loves his 8 yr old son matt dearly.i really want to develop a relationship with matt as i care for him too as i have been in his life since he was 2.the problem is tom's exwife feels i'm in "her spot" with tom(he left her 7yrs ago but not for me and she never got over it)now she has no control over this but has made it very clear i am to have nothing to do with her son matt.meaning i am not allowed to attend ANY school functions,spend time alone with matt,have no contact with teachers,doctors ect.she feels this is for her and tom only and i'm not really her son's family so its none of my business.tom being a great dad will not go against her wishies in fear it will cause problems for his son.my question is why all this drama?why can't she be happy her exhusband married someone who loves her child and wants to be part of his life?