For one, the dynamics of the known molecular pathways involved are complex [1], and so far non-trivial to manipulate.
Furthermore, even with an actionable mechanism, targeting the tumor itself is highly non-trivial, due to physiology alone. (See e.g. [2]).
[1] http://www.jcancer.org/v07p1497.htm
[2] http://www.nature.com/articles/nrclinonc.2016.119