Its a long story but in a nutshell the main reason it stopped was because it had served its purpose, which was to get the Ark shops well known and a huge success (which they are) Their is about 9 shops owned by Rob Tyrell (main promoter of Ark) and these make him so much money he just didnt need the hassle of the raves.
So not only did the Ark shops start to make much more money than the raves did, but by early 1997 when it did finish the club scene was changing, and they thought the best thing to do was finish on a high rather than drag it on and risk killing the whole ethos of it all. (this is what life should have done!)
Ark was run as a propper business and it did exactly what the promoters wanted it to do
Their is a few other things but those are the genuine main reasons.