So far, from what I hear about ConTeXt, it sounds like the solution to many of the major problems in LaTeX (perhaps all of those that can be solved at all). It supports grid typesetting (which is supposed to be very difficult in TeX --- no idea how the ConTeXt team managed it), absolute positioning of elements (layers), PDF/X support, XML parser etc. It is built from the ground up with a dynamic programming language (Lua), unlike LaTeX whose more limited expl3 language is not even finished yet. Many of the issues that LaTeX3 promises to solve in perhaps a decade from now have solutions right now over in ConTeXt.
People say its internals are not very well documented, but that is hardly an issue for an end-user like me. Also, the fact that it is still in development does not seem to affect its user interface very much.
But given the fact that ConTeXt still has a much smaller user base, something (other than just conservatism) must have prevented people from switching. Is there something LaTeX can do that ConTeXt cannot?
IMO, availability of packages cannot be the whole reason, since most users of LaTeX AFAIK stick to a small selection of "standard" packages that provide them with all of the most necessary features, and whose authors communicate with each other to avoid clashes. Might be that ConTeXt does not have packages for setting up Su Dokus, but that is hardly what makes people stick to LaTeX.
expl3in ConTeXt as well by doing\input expl3-generic.tex. It could be that some things behave strangely, because ConTeXt renames or moves some of the TeX primitives, but in some short random tests I did not notice anything odd. – Henri Menke Jun 20 '16 at 11:36