There are a number of reasons why Perry might want to attack Bush. First, Perry's greatest weakness is the fact that he resembles the unpopular former president: They were governors of the same state, Perry was appointed by Bush, and even their styles of speaking are similar. Many Republican voters are unreceptive by the idea of nominating another Texas governor. Other Republicans are more favorable to Perry, but fear that a Texas governor would not be electable in 2012. By attacking Bush, Perry might hope to draw a contrast, and distinguish himself from Bush. He knows that he must create his own identity, for he cannot win as a Bush clone.
Second, many in the Tea Party did see Bush (and his Republican Congress) as unfaithful to conservative principles. They feel Bush did not do enough to reign in government spending, and they were disturbed by various other Bush policies, such as the TARP bailout and Bush's proposed immigration reform. Perry could appeal to Tea Partiers by casting himself as a genuine conservative, and he cannot maintain conservative authenticity if he avoids criticizing Bush's policies.
What about the potential drawbacks to attacking Bush? Wouldn't he lose support among a vital constituency? After all, wouldn't Perry need all of the Bush supporters from Texas in his coalition?
Not really. Perry lost favor among Bush's inner circle years ago. The Bush crowd even attempted to end Perry's political career by supporting a Republican challenger (U.S. Senator Kay Bailey Hutchison) when Perry ran for reelection as Texas governor in 2010. Hutchison was highly favored in early polling and she became confident she would defeat Perry. As a result, she announced that she would resign her U.S. Senate seat prior to the gubernatorial election. Perry quickly announced that he would appoint Tea Party favorite Michael Williams as her replacement. (As governor, it would be Perry's job to appoint someone to fill the vacancy.) Williams would have been the only black U.S. Senator. However, Hutchison decided to renege on her pledge and remained in her seat. Perry dialed up his rhetoric and continued taking measures to appeal to the Tea Party crowd. When the Republican primary day arrived, Perry easily defeated Hutchison and was reelected.
Thus, not only did Perry win in Texas without Bush's supporters, but he won with them actively campaigning against him. If Perry ran, it's doubtful that Bush's inner circle would support him anyway; they are more likely to affiliate themselves with an establishment candidate like Mitt Romney.
Bush's supporters would be even less influential in states like Iowa and South Carolina than they are in Texas. Those early primary states are the ones that will decide Perry's fate. If he can win Tea Party support in those early contests by attacking Bush, he would be wise to do so.