Hello,
The language subcommittee only allows languages that have a living native community (except Wikisource, due to its archivist nature). This is based on an interpretation of the Wikimedia Foundation mission to "provide the sum of human knowledge to every human being". Thus, the overriding purpose of allowing a wiki in a new language is to make it accessible to more human beings. If a language has no native users, allowing a wiki in that language does not fit our mission because it does not make that project accessible to more human beings. Instead, a wiki in their native languages should be requested if it doesn't already exist.
Typically, the users requesting a wiki in an extinct language don't want to provide educational material to more people at all, but only want to promote or revive the language. While these are noble goals, they are not those of the Wikimedia Foundation, so that a wiki should not be created simply to fulfill them.
But that is my opinion. What do you think; should wikis be allowed in every extinct language?