This might seem obvious, but the chief allure of Minecraft and Roblox is pretty simple: they're video games, and kids love video games. They're also very easy to learn and the graphics are low-poly for both of these games, they don't need intensive graphics processors to run; therefore, they can be run on virtually any machine. Both of these make Minecraft and Roblox very appealing to a mass audience.
Kids have loved sandboxes for a very long time, and there is a reason why Roblox and Minecraft are known as "sandbox games." For kids as early as pre-school, sandboxes are an excellent way to use their hands, explore, play imaginary games, practice social skills, collaborate, and strengthen technical skills (e.g. fine motor). In Minecraft, all of these principles apply. There's a timeless allure to creating your own world as a child. In fact, this is a well-known tenant of Young Adult fiction: a main character, usually a child, who is whisked away into a fantasy world.
This was not an accident: Minecraft was built for this type of thing. The game encourages exploration and creativity. As a result, kids have made extraordinary things, and adults have modded the game to its logical end, creating custom objects and mechanics that had never existed before.
In Roblox, the sandbox principle still stands, but the worlds that are created can be much more flexible.
Learning is social, but a lot of socially-based learning projects (like standard group projects) force kids to do a project they don't want to do with a group they don't want to socialize with. This might be unavoidable in traditional classroom settings, but in Minecraft/Roblox it's not. The social aspect of these games operate similar to a school playground, for better or for worse. When these forces of pure fun and chaos can be re-directed toward education, the results can be hypothetically amazing.
On the other hand, it can be unfocused, chaotic, and, sometimes, mean. The strength of traditional group projects in a school setting mitigate these cons; they were, in effect, created to do so. But kids can be exclusionary and unwilling to push themselves (as adults can too), and this would have to be corrected by a teacher or leader.