Most of the beginners do not even know that the sandbox exists or what its purpose is. I myself learned its purpose only in mid January, less than one month ago, though I had seen it since around 2013.
In most StackExchange sites, people have incentive to simply post their questions and get answers. You only need 1 rep to be able to post questions and it does not cost anything. The sandbox's incentive is something like the opposite: questions need to get some sort of approval before being posted. This is not the way that StackExchange sites were designed. Additionally, beginners don't normally participate the meta, because the meta was designed to address questions about site organization issues, retagging, close and reopen votes, disputes and things that beginners know that they aren't mature enough yet to participate or even to understand what is going on, so they simply don't bother to come here.
There are a lot more problems in the sandboxes themselves. If a beginner randomly followed a link to a sandbox question, he will have no clue what is going on. Before I posted a comment to this sandbox question four days ago, none of them had a single word explaining the purpose of the sandbox, and the first one has been out since 2011. The best that I could get was the "Use this as a sandbox for new questions.", but no one was saying why should I do that.
In experience, the first time I saw the first sandbox, I thought:
WTF is this?
What is that Mk-roman-numeral thing? Should definitely have no relation with Mortal Kombat.
Why would I sandbox a question and how is this related to the Mk thing?
Well... no one seems to be explaining that, so it probably doesn't matter anyway.
I took quite some time to figure out what it was and what was its purpose. I eventually figured out its purpose only by seeing a handful of small, scattered, random comments citing the sandbox from random users on random questions and answers somewhere else very very far from the sandbox questions themselves.
And about the Mk thing, you (the OP, Quincunx) were the first person that posted something explaining what it is (at least the first post that I could find), and then you definitely convinced me that it surely has nothing to do with Mortal Kombat. As you explained, it means "Mark".
I conjecture that many beginners are thinking just like I myself thought; if they eventually see the sandbox, they would not understand what is happening. However most won't even see it, because the meta sites aren't usually for beginners.