Usurpers sympathetic because the heir is a jerk.
Usually, the Regent for Life is the bad guy. We say usually, because in these stories the rightful heir to the throne is usually a heroic figure, a nice if inexperienced youngster, or at least a decent guy who can actually claim legitimacy. While the usurper is portrayed as greedy, powerhungry and brutal, willing to exploit the regency to earn the prestige and influence to take power, at any cost. Problem is, sometimes the positions are reversed. The heir has the automatic advantage of legitimacy, but what if he's a monster? What if he's incompetent? Even if he is competent, what happens if the kingdom is facing a terrible crisis only an experienced and wily leader can face down, and allowing the rightful heir to take the throne would plunge everything into chaos? This is the (rare) situation in fiction where the rightful heir to the throne is absolutely not the person for the job. It can be the end of a regency (which now has, for the genuine sake of the realm, to be extended) or it can be the king dying and the heir turning out to be a childish charlatan or, even worse, actively malicious. In this situation, the other claimant has all the qualifications but none of the claim, making for a far more complex (and potentially grey) story. Seen it... not that often actually. I wonder if we need an index for rare tropes? This trope can also extend into the overthrow of an evil or incompetent monarch, but only cautiously, it has to be another monarch replacing it rather than a non-monarchial La Résistance movement.