Why is telluric acid dibasic?
Two may be as many protons that a Bronsted-Lowry base as strong as hydroxide ion can remove.  Building up more than two negative charges than that on the ion is a tough job.  Possibly a stronger base, such as amide ion in ammonia, could do that.

That the basicity of the medium could impact how many protons can dissociate is nicely demonstrated by citric acid.  Ordinarily it is triprotic in aqueous solution, but spectroscopic evidence indicates that at high alkali concentration (pH>14.4) a fourth, alcoholic proton can be removed.  There would be no doubt with a base such as amide in ammonia.