Resistance in democracy needs to be well-timed. Too early, and you don't have a critical mass of force to oppose the ruling regime and appear unreasonable. Too late, and it's too late to do anything.
This, plus you must consider the risks of taking bold action that can be framed as insurrectionary. I don't see a lot of haste to immediately and unequivocally declare the elected President, a usurper. Some would say the trouble is, the man got large numbers of legitimate votes under false pretenses, in a system where such an act is expected to lead to buyers' remorse and midterm losses through the political system.
Instead, we have whatever this is. Doesn't look like it's complying with the normal process of the political system, which is designed to punish an electoral bait-and-switch. Looks more like Russian elections and populace-management.
The right moment is only obvious in retrospect.