Because our first-past-the-post presidential system makes it so that it is exceptionally unlikely that any one not affiliated with the 2 largest parties can get any level of support at the federal level.

If we had a semi-parliamentary system where power was vested in the group that had the most votes in the assembly, you could see more jockeying for third parties at lower levels that still caucuses with one of the two primary parties. But as long as we have separate branches and a system where you have to get 50%+1 vote, we will only ever have 2 parties.