Because giving them their cutie-marks would reduce the material to work with for writing would be cut down a lot because that is their main goal.
If you compare that to Twilight, who is now a princess and no long a student; what goal does she have now and what little quantity could they possibly write about Twilight now?
So I don't mind them without their cutie-marks, but I do want to see them eventually grow up. But not right now.
which is why you do things periodically. giving one crusader a mark would be moving things forward a bit, showing that they are growing up, while continuing the same goal to achieve (there is episode material right there). the one who gets it wouldn't abandon them, and it would still be cause for adventures.
but marks aren't everything. they have other types of episodes too, scootaloo just had one involving her wings. sweetie belle has had one involving her sister. there is other material to work with. their childhood goal doesn't have to be completely achieved, but at least give them something that tells them they are getting closer (instead of them obliviously thinking some random thing is their life's calling). sweetie belle could still learn some magic, a milestone for sure. scootaloo's wing ability is now on hold once again, and i'm not really sure what apple bloom could get lol
twilight is somewhat in the same area too. she might have a new title, and student-teachings be finished, but that doesn't change much. celestia is still there to help her, she isn't a master at magic, and still has much to learn of life and friendship. i suppose her previous goal was simply learning friendship, and it has added to that over time. her current goal for the season is the box, but there is many other subjects to give her too.
there is much for both to do, the same old standard cutie mark crusader formula is losing its luster and they really should go for a new goal like they've given twilight. unfortunately they probably wont
