The reason is tied to why I don't consider myself or most of the people who maintain language blogs to be linguists in a formal sense. The word is commonly used to mean "multilingual" (or "polyglot"). And far be it from me, of course, to decry that usage. I would not want to have the descriptivist crowd on my case. And more importantly, the word has a venerable history being used as, and therefore meaning, "someone who speaks several languages." This history goes back at least to 1593, if the OED is to be trusted.
But in looking for linguistics blogs, I was not looking for blogs by mere multilinguals or polyglots. I was looking for blogs about the study of linguistics, preferably by people who are linguists in another usage/meaning of the term, people who study or preferably specialize in linguistics. In this sense, speaking two or more languages doesn't make anyone a linguist, and in this sense I don't consider myself a linguist because I don't have formal training in linguistics and therefore don't understand linguistic terminology and theories as well as someone who does have formal training.
To sum up, there are good blogs out there by multilinguals giving good ideas for language learning. But these blogs do not generally delve linguistically into how humans (best) learn languages or display knowledge of the research that has gone into the question. This is not a criticism. Simply an observation that when I searched for "linguistics blogs" or "blogs by linguists," I didn't find what I was looking for and, for myself, will be using the word "linguist" to refer to someone who understands linguistics well and "multilingual" for someone who has the awesome capacity to learn a bunch of languages.