Kovid's decision to develop the calibre editor was motivated, in part at least, by the fact that Sigil didn't have anyone maintaining it - i.e. it had been 'abandoned'.
I also recall he produced the first version remarkably quickly. Presumably because: he could leverage what calibre already had; he enjoyed doing it, or so he said at the time
; there were no legacy issues (i.e. a current user base); he could avoid Sigil's legacy issues - e.g. WYSIWYG Book Editor; and he could add some features - AZW3 support, multilingual spell checking, user functions in S&R, Live CSS tool etc, checkpoint/recovery.
IIRC the initial release of the calibre editor, and Kevin H announcing he would take Sigil under his wing happened within a few weeks of one another.
I've often wondered: if Sigil had been rescued sooner, would Kovid have developed the calibre editor when he did.
Nothing above should be read as me being critical of user-none, Kevin H or anyone else.
BR