Although I've never actually read the manga, I was always wondering why this CLAMP title (out of 3) has been put on hold for so long. So here's a snipit for that AnimeNewsNetwork interview, on what Ohkawa of CLAMP gives for an explanation:
" Speaking of X... I’ve been asked oh-so-many times (laugh). However, there are many social problems happening right now, in Japan and around the globe. Many people have died because of major earthquakes, plus there were several horrible murders committed by juveniles in Japan (note: Sakakibara Seito) – both are related to the story of X. Those reasons made it difficult to continue serializing in a shoujo manga magazine. Basically, we want to finish the work, but serializing it is difficult, so we are still looking for a suitable magazine to serialize it till the end."
(Rest of the interview can be found here).
...just if any fans were wondering about a reason.