Because (1) I only have a stock 400, and (2) I only have the BASIC Programming Language cart, and (3) nostalgia.
Of course there's newer stuff, but if that was the point why would anyone use any flavor of BASIC at all? I'm just tinkering with some old tech from my childhood and sharing the experience with my son. He's been having a lot of fun with my old 8-bit collection, and I want to take advantage of that before he outgrows it (like his brother did). I learned (like many of us) on the school's Apple II, and got a $50 C64 in high school. That's the extent of my BASIC experience. But I recently found a tough-as-nails 400, with an Atari BASIC cart, for super cheap, and figured this would be a great first computer my son could learn from and abuse. That being said, I'm just trying to overcome some of those common BASIC commands that aren't available in Atari such as INPUT (as I know it), TAB, and CLS. We're just writing text games right now, we'll move up to graphics later.
If anyone else is interested, the best Atari BASIC book for simple programs for kids I've found so far is this: http://www.atarimania.com/documents/41-Fun-Projects-For-The-Atari.pdf.