There are various factors including the unpredictable nature of the flow as well as the cost and sheer effort involved. But, also very prominently is the cultural sensitivity to not disrupt the will of the volcano goddess Pele.
http://hawaiitribune-herald.com/news/local-news/county-diver...
KHON2 (the local Honolulu Fox Affilate) has a good 5 part series on the ongoing Kilauea eruption -- check part 3 for more discussion on your question.