At some places they have. I know that George Washington University's adjuncts, at least in the School of Professional Studies, belong to an SEIU local. (If they want--the school is located in Virginia, which is a right-to-work state.) However, unionization drives do face many difficulties.
GWU is located in Washington, DC, which as far as I know, doesn't have "right to work" laws as neighbouring Virginia does.
I've often asked this question. Why doesn't every worker from every field form a union? It is always better to negotiate as a group then as an individual.