I suspect that is the case as well: outside of the more diehard fans who liked getting under the hood, the rules weren't streamlined or intuitive enough to let DMs just make a monster.
But on the other hand, I think the biggest pattern I did see is a lot less "get under the hood and tinker" support. Especially in the DMG. Less variant rules, less support for complicated rule creation (species or subclasses) etc. The areas they do focus on giving you tools for (adventure design, world-building, smaller rules like backgrounds and spells) still keep the game hemmed in at the borders. Ie: build an adventure or campaign using the tools we've given you vs make your own tools.
I suspect there are a couple of reasons why this is.. First, I suspect it is to reinforce a specific style of play (broad as it may be) as the D&D style and ceding other styles to 3pp games (which after 20 years and an unsuccessful attempt to roll back the OGL, they realize are never going away). In that regard, it feels a bit like 4e; focus on what they feel is the core game experience and cater to that. Second, I think that they realize anyone looking for an alternative to D&D has or will find it! So there is less need for D&D to provide its own kitbashing tools. Lastly, I think they are acknowledging that most DMs aren't actually good game designers, despite what they think. The casual players and DMs will be content using the prefab stuff with some minor edits and reflavoring, while the more hardcore and gifted well move into the realm of design and need/use far more sophisticated tools than the DMG could provide anyway.
I suspect there are a few other elements as well (it helps sell supplements and is easier to support using digital tools if there are finite limits) but I don't think those are the big reason for the shift. I just think it's an acknowledgement that D&D is a game and not a toolbox. It doesn't have to be able to support a variety of different things, it just needs to support the core D&D experience and variations there of.
And while some people may lament this narrowing of D&D's focus, I like to think of it as a gift to the 3pp community. They are ceding ground to other games to fill niches that D&D doesn't. Look at 5e Middle Earth, a game built on 5e assumptions but so radically different that you could never make it using D&D core books no matter how good the DM toolbox was.