laziness. Blue leds typically have a 3.3v drop across. 2 AA's gives you 3v.
I wanted 10 leds. So even if i was okay with a little less than 3.3v [these leds will light with about 2.9 minimum, but not very bright], i would have had to wire 10 leds in parallel.
Which would take up much more space and be super tedious. 18v gives me 5*3.3 + a little to spare, so i can wire two rows of 5. the extra 2 resistors weren't a big deal. MUCH easier than a load of 10 parallels.
[ed.] and I had the 9v hookups laying around, try taping together 2 AA batteries, and connecting them without some sort of snap on/snap in holder.