If the only way to reach you is to either install Signal or wait until tomorrow when they see you in person, people wait until tomorrow.

If the only way to reach you is to either install Signal or wait a year until the lockdowns are over, people install Signal.

For my wife, she had to travel abroad and the family had to stay in contact with her. Since she absolutely wouldn't use WhatsApp, they all installed Signal, and discovered it's actually really usable now.

I can only speak for why one company adopted Signal over WhatsApp, but the main reason was that the company did not want their communication metadata tracked by Facebook. They were regarded as equivalent in terms of E2E encryption and functionality.

EDIT: They also did not trust Facebook entirely not to break the E2E in some way (eg cloud backups or whatever), and the message contents had to remain secure. It wasn't a huge concern, but all else being equal, Signal was the better choice.