1) Because it is run by the scumbag Grover Norquist
2) Because it's a stupid pledge. What we need is tax reform. Any worthwhile tax reform will tend to lower rates overall, but also eliminate lots of deductions. Some of those people currently taking advantage of the deductions will end up paying higher overall taxes and whine and complain that the tax reformer violated his pledge.
Simplistic pledges sponsored by scumbags should not be entered into.
If it a stupid promise, then both Rand Paul & Ted Cruz are also stupid because they announced the same tax promise last week.