IMO it persists because there already have been concepts going into this direction. Some zooming trackers (Dream Station I/II, ReViSiT) went a step further, Aodix went a step further by drawing the note rectangles, Buzz replaced commands by only values so to say and didn't keep the order list as it was. So when it makes sense to shift away from Fast Tracker or whatever you consider being the tracker identity per se, it's nice to have and doesn't hurt anyone. E.g. the order list, the fact that in Renoise all tracks have their pattern end and start at the same point in time, is still there although e.g. Buzz abandoned it long time ago and it always looked senseful to me. If you prefer a traditoinal approach - it looks possible to keep the "big solid blocks" despite the DAW allowing you to have "overlapping clips". No one would lose much with a new possibility, although you can argue that you will lose support for the old work flow over time. Ironically, the order list does reflect the music I usually created. But as long as e.g. automation has these "stop and go" points, it's simply a bit weird to use, even if it has become merely a visual issue. Usually the curves spanned several patterns and so the issue was back in my mind again.
Also, it's too hard to record (not step-edit) for a longer time and then edit notes, partly because of the pattern length issue, partly because of delay column values that won't change without a lot more editing (alternatively have insanely high "zoom" forever via LPB), partly because you could lose off-commands when you try to "reorganize notes as objects" (maybe that concept should be seen as invalid in a tracker, I don't really know). Midi input has been there for a long time, so if there should be an idea how to improve it, put it in, it's currently like half way there. IMO, see "piano roll" as placeholder for things that could be easier, independend of what the solution would look like.
Renoise offers a very nice layout that you won't find in many piano roll based DAWs, e.g. FL Studio has been awful with the windows floating around for a long time. So, some people - at least I did - have picked Renoise not mainly because of the cells and commands, but because of some strengths that don't come from the tracker part, and for some strengs that are a consequence of the tracker part - most importantly intensive PC-keyboard usage.
My favorite reason not to want a tracker with piano roll view, as long as there is no good concept how to combine them, is this: When each column stands for each midi key (see Raul's piano roll concept http://forum.renoise...pattern-editor/), you would end up pressing the cursor keys a lot more. What in a piano roll DAW is the mouse moving a little bit would be you hitting cursor-right key up to ~11 times. Yet, it would be possible to let the cursor jump to a corresponding column when you press a virtual piano key on the PC-keyboard, which would be ok. In general, if a piano roll would mean I had to use only the mouse, or - worse - had to use a combination of mouse and keyboard, then I wouldn't like it.