Two of the main issues that are commonly cited (but are by no means exhaustive) are that:
Luther found immediate support and patronage from Frederick III, and then several other rulers of independent German states were also swayed to Protestantism. The initial support meant Luther had breathing room to expand his thoughts, and a safe space to spread it, and the fragmentation meant that it was harder to stamp out. Also, the large area covered by spoken German made it easier to spread the message than in Jus Hus's Moravia, for example. Luther consciously translated his Bible, for example, in as "neutral" a German as possible--in many ways Luther's Bible set the standard for German prose writing.
The printing press existed. Luther's 95 Theses were one of the first international scandals made possible by the printing press. Luther and the other early Reformers (as well as their adversaries in the Counter-Reformation) made extensive use of the printing press, a technology that wasn't really available even to Jan Hus a century earlier. Look at the meteoric rise of printing technology (Martin Luther was at the start of the 16th century, Jan Hus the start of the 15th) and then keep in mind most Protestantism wasn't spread through books but mass printed pamphlets, flyers, tracts, broadsides, etc. This let information travel in a way that just wasn't possible earlier.
Catharism is a rather hard thing to pin down. For a long time, many historians relied on the charter of the Council of Saint Felix as evidence for the existence of the Cathars. The problem is that the document that is purported to describe that Council is now considered to have very likely been a forgery. There is no evidence for widespread belief in dualism (the heresy that the supposed Cathars were supposedly committing) in Southern France during the time of the Albigensian Crusade and dualist beliefs do not commonly come up in Inquisitorial Records from the region. The word "Cathar" itself was not used by people in the region.
Whatever the variations of Christianity were practiced in Occitania were, there was certainly no organized Cathar Church or movement whatsoever. Most supposed Cathars were likely just non-Church sanctioned preachers (called "good men" or "good women") who preached in ways a bit unusual to the Church, and the people who associated with them. There was no Cathar leader (the enemies of the Albigensian Crusade were nobles such as Raymond Roger Trencavel and Raymond VII of Toulouse who for some reason didn't appreciate large Crusader armies showing up and trying to take their land), there was no organize Cathar belief, no Cathar texts or scriptures, et cetera. Catharism, as it is commonly pictured, just really wasn't a thing.
Here's a great answer by /u/idjet on the topic of Catharism.
