Signal is a much more neutral brand than RedPhone. RedPhone sounds either like an adult network or spying app.

At one of the Shuttleworth Foundation gatherings (where all Shuttleworth fellows meet bi-annually) I remember having a conversation about the branding of RedPhone and everyone unanimously agreed that it would hinder wider spread adoption of the software.

I'm not sure what all the factors in re-branding were but I remember Moxie agreeing with the sentiment at the time.