If they added a second circle pad to the 3DSXL, either it would never be used and it would be worthless and dumb, or it would be used, and then you'd have to buy both the original and the XL to get the full 3DS experience (dual-analog + actual portability). If they were going to add a second circle pad to a revision, it would have to be one the same size as the 3DS (or, like the SP, a drastically different, mostly for the better, design), and it would have to be out now, not a year or two from now, and also games would have to use it (the SP didn't have this problem -- the feature it added was a dramatically better screen, and every game already used the screen; there's not anywhere near that room for improvement on any one thing on the 3DS except the battery life), so they're just going to wait for the next generation.
Adding a GBA slot to the DSiXL wouldn't have been quite as dumb, but it would be the same thing basically -- either no new games would ever use it and it would be pretty much worthless as the relevance of GBA cartridges continued to dwindle, or it would be used by new games coming out with Guitar Hero DS-style Slot 2 accessories, in which case it would segment the market so that you'd have to buy the XL to play the new games, but you'd still need the Lite or the DSi if you wanted portability without carrying a purse (or a bigger purse than you maybe otherwise would have gone with).