Because those neglecting it are the same that want to remove it. So it's not “we want to remove it because it's neglected”, but “we want to remove it so we'll neglect it”. This is a pretty standard M.O. for the destruction of the commons.

If you look at the WHATWG GH issue, you'll see that two distinct, modern, maintained implementations of XSLT, one of which in Rust (so considerably less likely to be affected by memory bugs) have been proposed as alternatives to what's currently used in WebKit and Blink. The suggestions has been ignored without a motivation, because the neglect is the point.