Jeb lacks his father's Reagan coattails, or his brother George's kind-of charismatic appeal to make a credible tax promise, even counting the media pushing him. It's not just the Bush brand that has been damaged---the GOP in general is now burned due to its single minded focus on delivering ONLY on its hawkish policy promises. Reagan/GHW Bush ran on a triple platform of pro-growth, pro-defense, and pro-family agendas, but only delivered on two of those fronts (no major policy victories for social conservatives). GW Bush ran on the same three fronts, but only delivered on one (more wars, but no social victories, and he outspent Clinton 3 to 1).
Since the last decade going forward, national Republicans are by now only associated with no-limits foreign meddling, "no intervention we ever do is a mistake" type commitments, while caving to the Democrats or Big Business on everything else. This is not enough to energize frustrated conservative voters, who are not getting at least two thirds of what they want, and so are staying home. This is also not attracting new or independent voters either, who see the only thing the GOP really wants to do, upon winning the White House, is invade or bomb more countries. If they pick another milquetoast moderate like Bush for the nomination, most everyone knows we're in for amnesty, more wars, more taxes, and more new morality for years to come.