Because it's easier and I don't want to fry my $1500 laptop with some hair-brained project's DC-power spike. But I admit that's not really well thought through and an earlier version of this project was DC only. Also, the main inspiration for this project was DC only, as I mentioned in the article (but reposted here: https://arstechnica.com/gadgets/2015/08/op-ed-how-i-gave-up-...)