THere's tons of CLone officers and thousands of Jedi out in the galaxy. My guess is Palpatine personally contacted Cody, by doing Obi-wan first it would catch him by surprise, maybe Palpatine figured some of the strongest Jedi like Obi-wan and Yod might sense something in time, we see Yoda actually did. Thus he targeted Obi-wan first as he'd be one of the few Jedi likely to be able to bring Anakin back to the Light. Yoda was the more powerful threat, but Obi-wan the more personally dangerous threat.