From all stats I've seen, this is not the case at all - tablets (that is, all iPads, Android, Surafce devices, etc.) only make up about 4% of the market share, as compared to 45% or more for each computers and mobile phones, and tablets don't seem to be growing market share much at all. iPad sales specifically do keep increasing, so maybe it seems like they are more popular than they are, but overall, tablets in general are still very much a minority in the big picture. Now there could be a higher % of MUSIC users on iPads vs computers, but I doubt that is anywhere near 50%. So, statistically, if you had to choose to serve just one platform, Mac & Windows computers is the one to serve.
Now, that's not to say I would not love an iPad editor version myself! But I can see why Line 6 focuses on the computer platform for the initial focus. I would not find a phone-based editor very useful (screen too small), but I suppose if Line 6 did a tablet editor, they'd be able to also have it work on Apple/Android phones too.