It's the underdog principle. The Basques got swamped pretty early on in medieval history and they never really recovered to become even a regional power. It's more fun to see an underdog who failed historically rise to power than to see a historically successful power maintain the status quo or grow even more powerful.
A pretty clear example of this (outside of Navarra) is how during the CK2 days, everyone was quite excited about the Byzantine renaissance, because historically the Byzantines never caught a break after Alexius I. But once the Byzantines became a superpower to rival the Mongols, sometime around the 1400s, people started to hate on them, because they were rapidly becoming a big purple blob, no matter how fresh they'd seemed originally.
I imagine that if Navarra had conquered Iberia early on and expanded into North Africa its colonial ventures in America would be perceived less favorably.
There are exceptions to the rule of course; I doubt very few people ever get really excited about England, despite the fact that it was kind of a minor power for most of its history prior to 18th century. That's because in OTL we know England succeeds, even if (taken objectively) it does kind of look like an underdog.