One of the main things they have talked about is growing organized play by making it accessible: reducing the need for numerous books for newbies.
Makes sense, but it isn't without issues.
For a start, the +1 book rule is there pretty much to avoid excessive focus on optimized builds and possibly killer combos, although I don't think we're nowhere near the risks of 3e/PF and 4e!
But then reprinting stuff just because it's popular is pretty much against the whole idea of the +1 book limitation. Especially because "popular" might be not just because of concept coolness but sometimes also because of power, or at least I would expect that an otherwise weak option soon becomes unpopular. So if there is any character material that
might end up being mechanical exploitable, it's more probably the popular material rather than the unpopular.
We don't know for sure this was the criteria for reprinting... maybe it's the opposite, maybe they actually reprinted only stuff that they recognize as "safe" for combining with the new XGE material.
I also don't buy the idea of reducing the need for numerous books for newbies. Newbies should probably be encouraged to stick with the PHB until they aren't newbies anymore. If some of them want to jump ahead, it is more sensible
not to give them even more stuff in the extra book.
I am starting to think there can be other reasons for SCAG reprints...
At some point either Mearls or Crawford mentioned they were introducing a
change to the Swashbuckler, or did I daydream about this? I mean an actual (minor) revision of this subclass. What if this is actually the reason for the reprints? What if also the Sun Soul, Storm Sorcerer and Mastermind have been slightly updated?