I'm sorry that you heard this news. That sounds very frustrating considering the work you've put in to raise your GPA. In my previous job, I worked with students applying to school (though never with students applying to MSW programs) and I find that most admission offices will be forthcoming if you reach out directly. I'd try crafting a very polite email reiterating your interest in attending the school and explaining 1) any extenuating circumstances that led to your GPA a decade ago and 2) your coursework since then. Then, I'd ask directly if they could suggest a path forward for future admission cycles.
For future applications, you might also attach an addendum explaining the same (past grades and highlighting academic work since then) or you might touch upon this in your personal statement.
Dear [ADMISSION DEAN],
Thank you for reviewing my application. Although I am disappointed that I will not receive further consideration, I am grateful for you time and attention.
I am writing because I remain very interested in attending [SCHOOL] and am eager to take any steps that would make me a competitive applicant in the future. When I was an undergraduate student ten years ago, I achieved uneven grades due to [explanation]. Since then, I have worked hard to improve my record, achieving a 4.0 in the pre-requisite courses in which I have enrolled. [Are the pre-req courses at the school you're applying to? If so, it might help to list them.]
I understand the need to meet a minimum GPA requirement and I would be very grateful if you could suggest any work I might be able to do to bolster my application for future cycles.
***
That's just suggested language and you'd want it to be in your voice, but hopefully it helps a little. Good luck! It can be so frustrating, but I know so many students who end up at terrific programs after a disappointing admission cycle.