IIRC, when the Devs responded to this (early in the HOI4 development cycle, when they stated they were only doing the 1936 and 1939 campaigns), they said the analytics (from Steam) showed that most HOI III players (>75%) played the earliest start date (this seems to be true for all their games with multiple start dates), and of those that didn't the vast majority played the September, 1939 start date. Less than 5% played anything else.
Later they said that in HOI4 this trend continued and very few players play anything except the 1 January 1939 start. This probably explains why other than changes to make sure the 1939 scenario will load correctly, they haven't had major changes since the 1st or 2nd DLC.
Later they said that in HOI4 this trend continued and very few players play anything except the 1 January 1939 start. This probably explains why other than changes to make sure the 1939 scenario will load correctly, they haven't had major changes since the 1st or 2nd DLC.