There is a standard naming convention, and it predates the creation of Discovery. Voyager is VOY, and Enterprise is ENT. No one calls Voyager "STV", as that would cause confusion with Star Trek V, the movie. If you've ever used Memory Alpha or participated in a fan community like Daystrom you'll know that this has been standard for a long time. By extension, Discovery is DIS, Picard is PIC, and Prodigy is PRO.

DSC is a special case because it's used internally by the production (even shows up in the show itself once or twice) so some people have taken to using it, but it's not consistent with the other naming schemes we use so it's not standard. In fact, when it came out that Voyager was referred to internally as VGR, basically no one switched because everyone was so used to calling it VOY.