Because Kaz Ishii can't.
Ishii will fool you with the occasional decent outing, but when he blows up  it's so spectacular he tends to take the whole bullpen with him, which is not what we need before a day game.
Time to face facts: Ishii has had 11 starts and seven of them have been out-and-out bad. Sure, before tonight he hadn't walked more than two guys in his last four starts. But that doesn't mean they were good starts — in those four starts he gave up 5 ER, 5 ER, 3 ER (in 5.2 innings) and 3 ER (in 6.2 innings).
It's obvious Aaron Heilman could top that. More to the point, it's hard to think of a pitcher who couldn't. Jae Seo? I have ample faith he could beat 2-7 with a 5.68 ERA. Jason Scobie? Bob Keppel? Manny Aybar? Matt Ginter? Anybody got the phone number of James Baldwin's agent? Ishii's only purpose seems to be making The Manchurian Brave look good.
Oh well. Once again we put our trust in Pedro. Once again we try for .500. I tip my hat to the people who were still there when it was 6-1, raining hard and yet still piss-hot. I would have been tempted to skedaddle even if Ishii had been pitching a no-hitter.
P.S. Yes, Gary, we get it — you don't want to be traded. Enjoy your time covering all of right field and three-quarters of center, champ.