As the help center says, voting on meta is different (and more complicated). Part of it is the same:
Like normal Stack Exchange sites, Meta allows members to vote on questions and answers. For most posts, votes reflect the perceived usefulness: well-written, well-reasoned, well-researched posts tend to get more attention and more upvotes. Highly-voted and ...
While it would be great if people did leave explanation of downvotes, if there isn't one, we already have a default position as to what it means:
Generally there is the tooltip message:
For a question - This question does not show any research effort; it is unclear or not useful
For an answer - This answer is not useful
And additionally on meta for a ...
On meta, votes indicate agreement or disagreement, unlike on the main site where they mean you need a bath, are a living brain donor, and that a recent ancestor of yours was a chimpanzee. Note that no rep is gained or lost due to votes on meta posts.
A downvote therefore says "I disagree". Sometimes that's all that needs to be said.
It is certainly nice ...