Apart from Signal just generally doing a good job here, I see a few other possible factors:
* Signal doesn't see user content, so it doesn't have a content moderation team
* Signal is designed in such a way that it can't comply with most kinds of legal requests for user data, so it doesn't need a large team responding to those requests
* Signal gets some amount of pro-bono legal help, so it might not have as large an in-house legal team as other organizations
* Signal isn't trying to directly profit from user activity, so it doesn't need to study user activity or engagement metrics with a view towards profiting from them; similarly, it doesn't need to manage relationships with advertisers
* Similarly, it doesn't need to try hard to grow its user base (that would be desirable, but it doesn't necessarily increase revenue much)
* Similarly, it probably doesn't need to try hard to expand into other business areas
(I think these things are generally great. Yay Signal!)
From the company perspective, this is still an alright state of affairs, because even when investors get skittish and less overtly speculative, the company can still improve profit numbers by cutting excess staff. Meanwhile in times of plenty, the hiring of that glut of employees drives the company value higher due to the speculation that they're going to be able to do all the things.
It's dumb, but investing is often a web of self-fulfilling prophecies. If investors think a company will increase in share price, they buy, driving up the share price, allowing the company to sell shares at a higher price, giving them more money to grow.