They used Signal because they are lazy, incompetent, or don’t know or care about established protocols for secure communications (or all of those.)
Consequently, when what those protocols are supposed to do aren’t used, a failure occurs and someone takes the blame. However, that person knows nothing will happen because nobody will charge them and if they miraculously got charged, the Fascist In Chief would pardon them. So, there is no incentive to following established protocols.
But why were they sharing highly sensitive information over a private messaging app rather than using secure channels? The most likely explanation is that they wanted to evade accountability: texts between government officials are supposed to remain part of the record, while Signal texts can be and in this case were set to disappear. As Phillips O’Brien
notes, war planning aside, what the group chat reveals is top officials’ contempt for and hostility toward Europe; some of them opposed an operation against the Houthis because clearing the shipping lanes might help our (erstwhile?) allies.
So the disaster reflected both stupidity and bad intentions. And the same is true of other ongoing disasters, including the shockingly rapid collapse of the Social Security Administration.