I think it's a coding problem. Terry wrote QL, back in February 2000, to keep stats per question, so to keep track of stats by answer for multiple-choice is a rewrite, and it may be well nigh impossible at this juncture - - some questions are FITB and some are T/F and Y/N (routines would be needed to check the "status type" of each question). It may also be a question of space .. with 36 odd thousand quizzes, with (say) an average of 10 questions each, Terry'd need to create space for another 1.296m records (using an average of 90% as multiple choice questions per quiz), which would grow as more quizzes went online.
This question was raised by justawful in August 2001 in the thread
Alternate answers, so I can only presume it'd be a coding nightmare which'd slow the place down (with apologies to justawful as no-one seems to have answered the question). These are my thoughts, and they're speculation at best. We need Terry's input.
update above link to current version of infopop :
http://www.funtrivia.com/forums/ubbthreads.php/topics/108284/Alternate_answers#Post108284