Gonna go with "it's a marketing tactic" rather than a phase out. XL is more expensive. Leads to more profit.
You can see them doing this on other pages too- the XL is always in prime location for visualization. So if a kid gets on the site and sees the XL, it becomes more appealing than the 2DS. Words like "Full 3D experience" and putting it on the left side of the screen (since we read left-->right) are pushing out the XL more.
So yeah, marketing. If I was Nintendo, I'd want the XL to sell more.