The non-Advanced D&D rules sets were written as an intentional contrast to Advanced Dungeons & Dragons, with the purpose of providing an introduction and coherent ruleset for groups of people who had no experience with either RPGs or miniatures wargaming and who would be learning the game entirely by themselves rather than by individually joining an existing group of D&D players. This started with the 1977 'Blue Book' version of D&D written by Dr. Eric Holmes, a child psychologist, which attempted to describe the basic rules for the game in just 48 pages, limiting itself to a few character levels and a small number of monsters, treasures, etc.
The 1981 version of D&D built on this with a "Red Book' Basic Rulebook by Tom Moldvay that similarly described the basic rules for playing and enough information to handle dungeoneering and the first few levels of character development (with appropriate monsters, spells, etc.), but with 64 pages was able to offer far more in the way of explicit examples of how to play and extended descriptions of basic game mechanics. This was followed the same year by the Expert Rulebook written by David 'not yet Zeb' Cook, which covered character development through level 14 (with appropriate monsters, spells, etc.) as well as wilderness exploration and a few other topics relevant to this range of character levels.
Frank Mentzer had over 100 pages to work with in his 'Red Box' Basic Set for the 1983 version of D&D, and therefore had the luxury of offering more detailed walkthroughs for completely inexperienced players, starting with a 7-page 'first adventure' to introduce a few basic game concepts, followed by a 9-page choose-your-own-adventure-style 'solo adventure', and even a 9-page sample dungeon in the Dungeon Masters Rulebook. In the same year were released Mentzer's 'Blue Box' Expert Set with almost the same number of pages as the Cook Expert Rulebook (plus an adventure module for wilderness exploration) and his 'Green Box' Companion Set, taking characters through level 25, which was the first D&D product to include detailed rules on dominion management and mass warfare. This BECMI version of D&D was concluded by the 'Black Box' Master Set in 1985 and the 'Gold Box' Immortals Set in 1986, making it in total far complicated than any of the earlier versions of D&D except for AD&D, despite it paradoxically also including the gentlest introduction to role-playing games generally and D&D specifically.