In my opinion you have not really argued why it is neglected. As a starting point, they seem to spend roughly $35 million per year: https://projects.propublica.org/nonprofits/organizations/824506840. $14 million of those are for salaries, so I would be surprised if new features are strongly bottlenecked by money and talent.
I am just guessing on these issues, but my suspicion as for why some features such as “live location sharing” and “display past encrypted messages from the group chat you were not a member of but only just now joined” are not (yet) implemented because they do not fit well into Signal’s approach to security/privacy.
edit: the issue raised in this comment has been fixed