I don't know, but you may well be the only one who's
actively creating new L1 levels and still using LemEdit. I think Dullstar may still using LemEdit, partly because he doesn't run Windows, and he also has complains about Lemmix's UI.
It's probably a combination of you being more used to LemEdit, and the simple fact that Lemmix has not been updated once since its initial release. I'll have to go back to one of your old posts where you explain exactly which aspects of Lemmix you hate, since off the top of my head, I'm not actually really seeing
that much difference between the two. Two major differences off top of my head:
1) the setup required before Lemmix is usable. Yeah, no contest there, although part of it is due to Lemmix trying to be usable across multiple versions of Lemmings (original Lemmings, ONML, Xmas, CustLemm, etc.). I'd agree a better approach would be a way to at least default back to looking at the same directory the EXE's placed at (similar to LemEdit) for the necessary resources, like LemEdit.
2) LemEdit has a more usable menu bar than Lemmix, exposing more options at the top level compare to Lemmix. Though I suppose it's less of a concern if you're used to using keyboard shortcuts to bring up the important windows in Lemmix.
To me, I think the main strengths of Lemmix are as follows:
1) not requiring a DOSBox setup to use. Even if you're using the patched version so that it works in the DOSBox in the first place, some operations (eg. file loading and saving) are still rather slow. [Of course, this is also a disadvantage if you are not running Windows.]
2) cut and paste and multi-select support (though the latter could use some improvement)
3) zoom
4) seamlessly go between playtesting and level editing
I suppose one of these days I could try to update Lemmix (not sure if Erik ever gave me the missing files though), but given that currently you are the only one actively creating new levels and you aren't even using it, you can see why it's hard to find the motivation.