Much of that is intertwined.

The biggest key imo is captured media (and more recently, social media), which has indeed been happening for decades. This leads to uninformed voters and manufactured arguments, groupthink, and tribal narrowmindedness, allowing the two parties of capital interests to remain dominant. Voters cave too easily to the "lesser of two evils" thinking, hence the race to the bottom where only capital interests win. The primary process is a complete joke where the dominant parties have full control over it and blatantly manipulate it (or skip it entirely like we saw recently), and running under a third party is suicide.

However, don't be fooled into thinking we just need >2 viable parties, as the same fundamental problems can easily scale if the other sources of corruption (media) remain. We see this in the UK etc.

Election fraud itself is probably not a very big factor in most cases. With all of the above, it doesn't need to be.