I find the x1 is easier to read at a glance. Probably the reason the Shearwater added colors to their screen. I have never used the Shearwater but I know that the X1 has a one beer learning curve. That is you sit donw with a beer and start tapping away, by the time your finished you can work about 90% of the functions.
I also like the fact that it runs vpm. I plan my dives on v-planner, its nice to know the computer you are using for backup is running the same algorithim.
Either way both computers are great and it really comes down to personal preferance. I would also venture a guess it has something to do with what their friends were diving at the time or their LDS was selling and most poeple have never actually tied both.
What I will say for a fact is that Liquivisons customer service is out of this world. I'm currently diving in Thailand for about 6 months. Couple months ago my x1 crapped out on me, (it happens to every maufactuer once in a while). Couple phone calls later they were Fed Exing me a new computer to Thailand on their dime, didnt even ask for the old unit back. Now you cant beat that kind of service. Well I guess he could have flown here himself and strapped it to my wrist