I assume they haven’t put much manpower into it. There are various open source components that will handle most of the federation for them as long as they submit their posts in the right format.
I don’t really blame Tumblr, but I don’t think they’re this slow because they’re doing it right. I think a small team of developers passionate about the idea are given some time to work on federation, but it’s clearly not a priority.
And honestly, how could it be a priority? Joining the Fediverse means opening up your content and platform for free, with no data mining capabilities or ads to support the extra load.