The Minecraft thing is, as others have said, largely probably due to the YouTube's changes, and the game's age probably contributes to it.

As far as Mindcrack goes, I think there were/are a few things involved.

The biggest issue is that Mindcrack was a Minecraft server, and they largely left Minecraft and the server. Despite the branding they've tried to develop over time, it was known for being a Minecraft server, and when they left Minecraft, they left the game most of their viewers cared about. And anybody who looked at any of their channels knew exactly what would happen if they decided to leave the game and the server. Back when things were going well, you could look at almost any channel and see something like: [Minecraft/Mindcrack video] 50,000 views, [other game video] 5,000 views.

Another big thing is that the group stopped being or at least feeling like a community. When things were going well, they all seemed to have some idea of what others were doing, but at this point there are so many people doing so many things that I don't imagine that anybody with any sort of a life could even keep up with half of them. (Unless things have changed since the last time I checked out any of their videos.) At one point, even if they didn't make videos together, there was still a sense that they were playing the game together in a way (which is also an important thing for the popularity of Minecraft server videos).

On top of everything, them losing guys like BDoubleO and Etho didn't help anything. The subreddit has continually lost subscribers since then (from ~52k to ~43k over the last couple of years).

It's understandable why they left Minecraft after years of playing it, but it's also the game (and server) that many people watched them for.