But Gygax and others often made sure to explain that AD&D was a completely different game than OD&D and BD&D.
AD&D was originally intended to be an expansion of OD&D. Notably the AD&D Monster Manual was published first, with the intention that people would start using these monsters and rules in their OD&D campaigns.
Then it became a way to not pay Arneson royalties and it was suddenly a "completely different game".
In addition, my understanding is that the DMG was delayed because it took much longer to finish than Gygax originally intended. Or possibly because of a problem with scheduling the printers. Or possibly both.
And then there were six AD&D modules published in 1978, before the game system publication was complete. (These were all billed as “ADVANCED DUNGEONS & DRAGONS” modules.) Those first modules published for AD&D were for high levels – 8-14.
This was, again, because these modules were being marketed to the existing OD&D market. Or possibly because the campaigns of the people designing them had been around long enough that their PCs were at mid-to-high level and, therefore, that's the material they were developing.
In '79 when AD&D is a completed ruleset, you get an introductory product. In '80 you see publication of the modules that were developed for mid-level convention play. Then in '81, as the D&D fad is beginning to boom, TSR rolls out even more low-level support.
It's not the result of a comprehensive marketing plan. But it makes a good deal of sense when you look at who TSR was actually selling to at any given time.
The other thing to understand about this is that TSR was actually late in coming to the module market: Judges Guild had already published a ton of material (and it was the success of that material that convinced TSR to start publishing modules). So TSR's product line is not the totality of D&D/AD&D product at the time.