SOCOM is more than SEALs
From what I've heard and read, the SEAL teams never really gave up the Sig 226. The HK mk23 is a SOCOM gun and as such, was created for Delta Force and other units (including SEALS) under the SOCOM umbrella. The HK is big, but it provides a good platform when a silenced, highly accurate shot is needed. The .45 acp is superior for silencing because it is subsonic.
The SEALs may have decided that need was uncommon as they have silenced 9mm HK mp5s in there arsenals.
Yo