For the same reason that someone would go with 9 speed when their 7 or 8 speed stuff started to fail: because it's what was currently in production at the time and eventually it became easier to find parts for, even if they could care less about having an extra cog on the cassette. Path of least resistance and all that. We're not quite there yet in the 9-10 speed transition, IMHO. But as the OP found out, finding 9-speed STI levers above Sora or Tiagra level is starting to become a problem. If I can find a good deal on 9-speed barcons I may pick up a spare set myself (though I think we have a long way to go before even those become scarce).