Politics at the top is the preserve of the wealthy and successful and encourages compromise and collegiality, so that there is little difference between a Clinton and a Giuliani, nor could there be since politicians - and this would also be true of any third party under capitalism - ultimately report to the bond markets. On the other hand, as we also know, there are very deep political, social, and cultural divisions and goals which divide Democratic activists and supporters from their Republican counterparts.
The differences which exist lower down would be more apparent to those who think it's important to track and be involved in political activity at this level, while the similarities at the top would be at the forefront for individuals and groups who are yearning for a new party and who effectively abstain from mass politics - often counterposing the latter to the need to "build the movements", as though the activities were unrelated.
In other words, the two groups talk well past each other on the issue, which inevitably leads to exasperation.