Just out of curiosity, why is B/X the (current) focus of the OSR community? Why not AD&D or 2E or BECMI? What makes B/X (and by extension OSE) the primary tool for folks dipping into Old School D&D?
I said "current" because I remember when it first started with OSRIC and that dominated initially.
I think the primary reason is that the bare-bonedness of B/X leaves the least number of jutting surfaces which might conflict with whatever the OSR-product-maker is grafting on to traditional D&D.
As you say, the initial OSR push was for retroclones like OSRIC. However, those have all been made*. Now, most OSR game that are being made are
'old school D&D, but with <some hook>'. That hook can be genre changes or expansive worldbuilding rules or a new interpretation on thief skills or name-level domain management or whatever the designer finds most interesting to play around with. It's possible that those things that oD&D or AD&D or BECMI have that B/X does not will work just fine with this new thing, but they might not. Certainly whatever purpose they served or were intended to address are likely to have been changed by the hook, so their purpose in the game is going to have to be re-assessed. Easier, perhaps, just to use the B/X framework and add back any AD&D-isms one finds appealing.
*with the occasional 'no, this is the real iconic <D&D version X>-with-the-serial-numbers-filed -off. I used a flat coarse file instead of a half-round bastard file'
I've never been clear why B/X is preferred over BECMI. My current working theory is that the "CMI" part, with its focus on domain-level play, the quest for Immortality, and then playing an Immortal, clashes with the low-fantasy aesthetic that the OSR prefers.
I don't know if it is the aesthetics (why vary widely in the OSR), or it is simply an existing hook that again conflicts with whatever hook the developer is choosing for this specific OSR system. If I'm making a game that explains what a character wants to do with themselves after the whole 'crawl through dungeons (later wilderness) for fame and fortune' phase, then I don't have much use for the domain rules or quest for immortality.
I'll also say that, and I say this as someone who has played more BECMI than any other type of TSR-era A/D&D, there are parts of it that are... less exciting than others. Of the CMI portion, the C-set domain level play (and associated seige&stronghold, mass combat, and related rules) seems... well, it seems like it was included because someone thought someone might need it. It doesn't feel especially evocative or engaging, and that's often where our campaigns petered out (or we went far off-script, with house rules and new ideas about what gaming at that level would look like*). The quest for immortality is conceptually exciting, often reinvigorating our campaigns. The limit there was that it was extremely DM/group-dependent, with very little rules structure (so something an enterprising developer might want to put their own spin on). Once you got to actually being Immortals, we never really were all that clear what an immortal PC was supposed to do, or even want.
*perhaps something to make an OSR game around.