Throughout this thread there has been consistent discussion about the PIC being able to see his wingtip. Not for one moment do I think this is the issue.
The truth hinges on what that captain saw, say, 150M prior to the collision, and how he interpreted it.
The worst scenario would be that he made the assumption the CRJ was progressing normally and therefore going to 'clear'.
Did he simply not see it? Remember, he may have been really straining to see the poor markings, and concentrating on keeping dead-center on the narrow and damp taxiway.
Only an in-depth analysis of this poor guy's visual clues and thought process leading up to the collision, will be of help to finding the cause.
Lastly, I found the communications bewildering. Is there a prize for being laconic on American RT? Converting from ATPL to ATP years ago, the radio was my only difficulty. It doesn't seem to have got much better.