Could be a few different reasons why your crown came off. I am not sure if this is the case with your crown but you may want to ask your dentist. It is not uncommon for *some* dentists to cement a premanent crown with temporary cement following a root canal treated tooth (specially if the crown was made very soon after the root canal). Reason for this is if they have to go back in and redo part or all of the root canal, they can access the tooth by easily removing the crown. Only option to retreat a root canal with a permanently cemented crown is to hope that the crown doesn't break when they try to tap it off or drill right through the middle of it then refill it with either composite material or amalgam and it would be a shame to do that to a new crown. Of course if this is what they did, 6 months is too long a time to leave a crown on with temporary cement. That and if the root canal wasn't a success, more than likely you would have already known about it in 6 months.
Other reason that it could be is that they didn't get the tooth completely clean and DRY when they cemented the crown on. Any contaminants present when they cement the crown on will weaken the bond of the cement they use. Another reason could be that maybe they didn't have enough cement in there to begin with. I know it sounds odd or funny or whatever but it does happen that the dentist's assistant may have not mixed up enough and instead of re-prepping the tooth for cementing and mixing up a new batch that they just try to spread out the little they got.
You already stated that you were in no pain with this crown after they cemented it in so that means that the bite on that crown wasn't high so that rules that out. What about the interproximals of it, how was that fit? Did you have any problems of food easily getting trapped in between this crown and the teeth adjacent to it? When you floss in between the crown and adjacent teeth, does the floss snap in? If you don't have a problem with food getting trapped in that area and the floss snaps in between as opposed to just sliding in with no contact with the adjacent teeth, then that means the crown is fitted right. The only other way the crown could not be fitted right is if the margins were off, and that they should have checked before cementing it in. If the margins weren't right and they cemented it in anyway, well all I can say is after this is fixed find another dentist.
In any case, your dentist should replace the crown for you at no charge seeing as how it is only 6 months old. As someone has stated already, if the lab made the crown according to the dentist's impressions they sent over then the lab may not remake the crown for free. In any case, the dentist should not charge you for that. Besides, if the lab didn't make the crown according to the dentist's impressions, then he/she SHOULD know that when they try to fit it in. The slightest variation from the impression could be told when they test fit it and check for margins.
Hope all works out for you.