It's basically because the name you used to have doesn't really matter, it tells nothing about who you are. Some ask it out of curiosity, which is invasive even if it's innocent (often that name reminds you about things you'd rather not remember), but some ask it just so they can use it to misgender you, and you can basically never know for sure is it malicious or not. Even people you thought were your friends might use the name later to hurt you if they get angry or some shit. So the best course of action is to never reveal your deadname to new people you meet. It's gone, it's past, it doesn't matter and nobody should care.
Even if you are cis-passing (nobody can tell you're trans just by looking), being trans means you have to either hide parts of you, or you are kind of fighting a constant battle of people reacting to you being trans. Even if you're in a position good enough for it to not be that malicious, it will wear you down eventually. You might think you're strong enough, that you can casually talk about your past and the name you had, you can take it! You're not a victim! And sure, maybe you can keep that up for couple years if you're lucky, but it gets old fast, and it will wear you down. Telling your deadname is just giving more free ammo to people who are more likely to shoot at you than you'd like