I do agree that SPLC and Stormfront do not compare, but it could be argued that Reddit blocks or suppresses conservative, non-multiculturalist views much more than liberal ones. That asymmetry is part of what fuels the growing extremism IMO.
Conservatism goes beyond states rights and economics. It's also cultural. Conservatives tend not to like multiculturalism or cultural change, and tend to like things like gender roles, immigration tests, and just generally keeping things the way they were. Rational and compassionate people can have these opinions too, yet they are suppressed on pretty much all social media sites including Reddit.
When people with moderate cultural views and opinions get suppressed, they cling to the nearest groups with the closest mindset to their own. Unfortunately, on sites like Reddit, these are often extremist subreddits.
/r/The_Donald has broken many rules of Reddit and the largest punishment they've been given is that /r/popular was created to steer people to less toxic content.
You can also look to how long they let /r/physical_removal (advocates of political violence towards "commies" and "leftists") and /r/incels (advocates of sexual assault, rape, and violence against women) stay open.
/r/MensRights is still around despite doxxing and harassing people in the past.
For the sake of comparison, here is the first paragraph of the Wikipedia entry for each:
Stormfront is a white nationalist, white supremacist and neo-Nazi Internet forum, and the Web's first major racial hate site.
The Southern Poverty Law Center (SPLC) is an American nonprofit legal advocacy organization specializing in civil rights and public interest litigation. Based in Montgomery, Alabama, it is noted for its successful legal cases against white supremacist groups, its classification of hate groups and other extremist organizations, and for promoting tolerance education programs.