There's a lot of reasons to think that national polling is not predictive of a primary race
- First, the relevant decisions are made state-by-state.
- Second, the sampling issue for primary voters is much harder that for general election voters. Among other reasons, people are paying a lot less attention, so people who care strongly are probably more over-represented than is typical.

Does winning the [Iowa and New Hampshire] primaries not make him the Republican party candidate? Are there other primaries that aren't being bet on?

Yes to your second question. Also, winning one or both of the early primaries is not a strong predictor of who will be nominated in contested primaries.