A few reasons.
Firstly, as said, Reddit is far more segmented. Reddit has a bad rep as a hivemind of memes and shit reposts, but it doesn't have a reputation as a gender war haven because, well, it isn't. Sure, r/whatever may subscribe to that, but you can also find r/theoppositeofwhatever, and its probably far more popular.
Tumblr doesn't have this same ease, because its individual users who do this, and there are a reasonable number of them. Rather than being able to just lump it in as "r/x is shit", you'd have to list them off as "user x, y, z, m, n, o, & p are shit", and that's way too much effort. Tags can help, but again, being completely user defined and not moderated it ends up just being a bit of a mess.
The structure also works to isolate these incidents from those who aren't looking for them. Lets take the JonTron controversy, and see what would happen if he was on Reddit, or Tumblr, instead of Youtube. On Tumblr, people follow JonTron the user for his gaming content, subscribing to him after seeing some of his posts on the gaming tag. He then posts a bunch of racist crap on the politics tag, and everyone who's subscribed to him sees it.
In reddit, you don't do this. You subscribe to r/gaming, and when u/JonTron posts something in r/politics... Nobody notices because nobody is notified. It keeps it much more under wraps by following a topic than following a user. It also labels its user following function, which is much harder to find simply because who the hell clicks on a user's profile on reddit [As opposed to a general blog on Tumblr], and on Reddit its called a 'friend' - which has a completely different connotation that causes many people to avoid it, even if it is literally just a 'follow this person and see their posts in r/friends and r/friendscomments' button.
Another piece to the puzzle is the types of comments. General sexism? Its fairly old hat, and its fairly well known for existing in some form everywhere. Even here on the Escapist you sometimes get posts hinting at that sort of thing in the political section, though often either only implied to avoid Modwrath, or having incurred Modwrath. Hence, it seems... wrong, to label general sexism as "Reddit's Thing", when everywhere does it. Conversely, on Tumblr, we have that new sort of ultra-feminist, which is kind of exotic and new and "Ooh, I ain't seen that before". Hence, it gets labelled big time, for being a relatively new thing. While other websites do have posters who are undoubtedly similar to these people on Tumblr, Tumblr seems to be the only place actually accepting of "I am actually a helicopterkin and have 125 genders, preferring the pronounces Kzhe and Kzhim", and taking it seriously, thanks to just being a collection of user blogs making it easier for people to just form an unmoderated group about this stuff, whereas on other websites its often more seen as "Dafuq?" and isn't taken seriously by almost anyone, with these websites instead being open messaging boards where you can't just pick a group of friends and reinforce your own fantasy in isolation, and instead have to put up with the ridicule of the general internet.
From a purely anecdotal standpoint, the userbase probably also affects it. Literally the only people I know who use Tumblr are <20 year old girls. Half the people I meet day to day use Reddit for some reason or other. Reddit has a much broader user base in my experience, and thus the 'special snowflake' types that everyone loves to ridicule, as well as the hyper-racist types, tend to just get drowned out in your general cacophony, and if you try to generalise a redditor, you constantly run into surprises when you find that attractive 26 year old female uni student sitting next to you browses it, as does your 45 year old male boss. Tumblr? When the only types of people you see using it day to day are the types of people closest related to the whole "I'm a unicornkin" craze, then its much easier to just label the entire site as that.
As said, purely anecdotal, and I'm sure there are a variety of users who use Tumblr, I've just never met them, and I'd hazard a much larger portion of its userbase is the somewhat self absorbed <20 year old looking to make their own personal blog - its kind of the whole point of the site afterall. This drastically alters the type of content you're likely to find on Tumblr, and hence it gets a reputation for that content.
More anecdote; logging out, going incognito, and activating a VPN browsing just the front pages of Tumblr and Reddit ATM:
Tumblr; A post about Club Penguin, a post about racism, 4 memes, a PC vs Console post, a music post, several poetry posts, a bunch of fandom posts, a nice landscape picture, and a bunch of that 'edgy' life inspiration quote stuff that's really a crock of nonsense.
Reddit; News, a post thanking someone for intervening to help his daughter who was being sexually harassed, some sport, pet pictures and memes, in-jokes, normal jokes, more memes and pet pictures, and more news.
The feel of the two is naturally very different. Tumblr feels like an angst ridden teenage cespit, Reddit feels relatively neutral, at least from just the front page of both. Its easy to see then why someone that takes a quick look at each might just believe and repeat the 'Tumblr is crazies' stereotype while not seeing it apply to Reddit.
As other have said though, Reddit does have a negative reputation, especially some subreddits. But it tries to hide and segregate it, and IMO has a much more varied user base. Tumblr just throws everything against a wall, and doesn't try to hide any of it, with a userbase that just feels much more angsty. Its any wonder one gets a reputation while the other doesn't.
