While there was a desire to expand the rules, AD&D was intended to function as a cutout of Dave Arneson from royalties.
Naturally that doesn't make for good ad copy, so the reason given was to "upgrade" and "enhance" the system, compiling everything related to each subject (character creations, monsters, rules for the DM) into one, very well put together hardcover (and the covers of the first ones were quite impressive).
Later, an additional reason, that of establishing a "standard" set of rules for tournament play was asserted. (That assertion went over . . . poorly, and contributed to a significant disdain for Gygax personally and the company in general among players.)
Because of the changes between O(riginal)D&D and AD&D, the game was MM was usable with only minor quirks with the old rules. (The biggest changes was the worst AC being 10 instead of 9.) It was also thus the easiest to produce, basically being just editing and expanded formatting.
Things get a little more divergent with the PHB, and there was considerable unrest waiting for the DMG. Fortunately for all us raging nerds, TSR published a "preview" in The Dragon #22 (it still had the preposition back then) containing the combat tables (including the save tables) and some magic item previews.
The DMG contained significant text on background and development concepts, and its 230 (or so, it gained a few with repeat printings) was beyond humongous at the time for a "wargame". (Which is what people treated it as, albeit an exceptionally quirky one.) (100 pages of "vehicle" (monster) charts was also huge, but people loved that kind of "detail" for their wargames. The PHB was "really" only 42 pages of "nation" (character class) charts and basic rules with 60 pages of more "vehicle" (spells) modifications, and so also was "big", but nowhere near the class of the DMG.)
I don't recall the specific details, but I vaguely recall something about the "Monstrous" books being a combination between ad hype for the size (particularly for the Manual as a single book, though the Compendiums had that "loose leaf" shtick going so you'd put them together into a single tome), and a bit of arrogance in "improving" the language and reading level in the books. (Gygax used both the "archaic", Latin-derived English grammar of the pre-1950s, combined with a vocabulary inspired by the insanely florid and baroque style of Jack Vance. If you think Vancian magic is a pain, try reading Vancian text sometime.)