Name recognition - of course they lead the field.
There is a built-in advantage that comes with the name Romney/Bush.
Very true. People just go with the name they recognize in these early polls.
But being governor of a sizeable state isn't the worst preparation for running for president and governing from the White House. Governors actually have to administer things. They have to make appointments and monitor the work of their subordinates and take the responsibility or blame when things go wrong. They actually have to make and balance budgets and convince legislatures to pass them, which is harder than just voting for or against a budget or holding up the process.
I'm not crazy about Christie or Romney and don't think I could vote for Bush, but if you really don't like them and want to beat them, you have to come up with a candidate who may be more like them than Herman Cain or Michelle Bachmann or Ben Carson or Rick Santorum. You have to have somebody who has actually run something. The irony is that Ronald Reagan was very much in that old tradition of governors as presidential candidates -- he was governor of the largest and wealthiest state for 8 years -- but his name survives now as an example of the ideological tribune as presidential candidate, rather than as someone who had extensive experience governing a major state.