In "The Fediverse Challenge" I quoted Moxie's reason for not making Signal decentralized:
"We got to the first production version of IP, and have been trying for the past 20 years to switch to a second production version of IP with limited success. We got to HTTP version 1.1 in 1997, and have been stuck there until now. Likewise, SMTP, IRC, DNS, XMPP, are all similarly frozen in time circa the late 1990s"
(Not saying that is a valid reason)
https://socialhub.activitypub.rocks/t/the-fediverse-challenge/1968