Are you using a dataset or a subject area? Explain works on datasets, it isn't available on a subject area (as there you should already know everything about your data).

A better reason for it is that a dataset is a very limited scope compared to a subject area: a dataset is a unique object, with a known and generally small number of columns (10-50). Statistics and correlation between columns can easily be calculated because it also uses the attribute vs measure logic to show a measure by attributes or an attribute by measure in various cases.

A subject area is generally multiple database tables, can have many dimensions and each dimension has many attributes. It's a lot bigger in size and as you could even have some switching logic, columns can change source based on the requests. It would be too risky to start doing the operations that "Explain" does on that.

You can still use Explain on top of a subject area if you want, but first you need to create a dataset based on that subject area. And because at that point you took a very complex object (your subject area) and you reduced it to a "flat" set of columns (probably a lot smaller), "Explain" will be available on that dataset.