Many people feel they have more to fear from government doing something wrong than they have to gain from government doing something right. So it is possible they will prefer whichever candidate seems the more modest and risk averse. That is how McCain is pitching his tax plan. Furthermore, this is a conservative era. It is conservative in the old-fashioned sense.
People cherish the status quo and want to preserve it.
McCain goes out of his way to say that his is the true conservative plan, and by the old definition of conservative he is right. Conservative in temperament, these voters may reject ambitious plans conceived in some think tank, and gravitate instead toward the candidate who is criticized for being too modest. They are looking for modesty. Of course GOP primary voters may be different. These are the people who cheered on the Republican revolution, the ones who listen to Rush.