This Wall Street Journal piece says the reason is fear of the racist charge:
But Sen. McCain vetoed proposals to attack the Illinois senator for his 20 years as a member of the church led by Rev. Wright, whose harsh comments about racism in America and other issues created problems for Sen. Obama during the Democratic primary contest. Sen. Obama publicly severed ties with Rev. Wright earlier this year.
Sen. McCain has said Rev. Wright is off limits.
That decision, and the worry that the campaign could open itself to accusations of racism, has kept Rev. Wright out of their strategy.
One McCain senior adviser said the difference between Mr. Ayers and Rev. Wright isn't race, it's religion. "It's not appropriate to attack someone's faith," he said.
I'd say it also has to do with Sarah Palin's dubious religious associations--if you're reading between the lines, you might even interpret that last quote that way. Amy Sullivan (aka my wife) gets into this in her new piece on Palin's Pentecostalism.