From 1935 to 1962, the US Navy used an aircraft type called the patrol bomber.
The aircraft that falls under this aircraft type and is assigned the code Y, that is, the aircraft manufactured by Consolidated Aircraft Corporation, is what you call PBY Catalina.
However, there are well over 30 aircraft manufacturers that existed at the same time in the United States alone, and there are 7 other companys even if limited to the same aircraft type.
Moreover, in fact, there are 5 other aircraft types with similar concepts.
Specific instances are Torpedo ( T: 1922–1935 ), Torpedo bomber ( TB: 1935–1946 ), Bomber torpedo ( BT: 1942–1945 ), Torpedo scout ( TS: 1943–1946 ), Patrol torpedo bomber ( PTB : 1937–1962 ).

It's a sorry that PBY Catalina wasn't chosen from them, but if any reason is given, that might be just only an excuse.
I think the real reason is that there are too many candidates.