People do not quit because of first time bonus, or 1st time attending the instance, they quit when the players are not geared enough or competent enough to learn when explained. there are people who simply put with their gear and skill level will simply not be able to complete some dungeons (or if do its as the timer hits 0, wasting a lot of time).

I have run with many 1st timers in AK and 90% of them are fine, but some are simply not even in AF, lv30 accesories, and have no clue how to play (thank you FATE grind). in these situations its realy not worth the time. Now geared correct and seems competent you go to first boss and see what happens (maybe stick out to the wall if are unsure), but in most cases you know at 1st boss what your chances are.

point is, its not that people are new, its people who are new and also do not have a clue about how to play that cause the drops.