Since question 1 has adequate answers elsewhere, I'll focus on question 2.
'Popular success' is difficult to analyze: it's a social phenomenon that doesn't correlate well with any overt aesthetic or intellectual values. 'Popular success' is as much a matter of zeitgeist (ironic nod to Hegel) as anything else. People like things that appeal to their current sensibilities. In philosophy in particular, people are drawn to ideas that echo what they are already feeling: ideas that put into words what people have already been struggling with.
With that in mind, there are two factors that probably contributed to Nietzsche's comparatively large footprint:
- Nietzsche was writing in German; Kierkegaard in Danish. Germany has always been recognized for a premier academic standing, so German is a natural second-language choice for anyone studying philosophy. Further, translations are laborious and expensive, particularly in the 19th century. Publishers would not undertake them unless they had good reason to believe a work will 'resonate' with a larger, trans-lingual audience. Kierkegaard started at a disadvantage.
- Even though Kierkegaard and Nietzsche are similar in outlook and tone, Kierkegaard was fundamentally religious where Nietzsche was fundamentally anti-religious. The late 19th, early 20th century — the fin de siècle era of Darwin, Freud, and Marx — was the beginning of the intellectual push against the social and political dominance of religious ideation. Revolution was in the air, not reform, and I expect Kierkegaard would have been seen as somewhat regressive, at least outside of Denmark. Neitzsche captured that inchoate sense of social unrest better than Kierkegaard did.