it's not a 'lack of police capacity in the westbank', it is state policy. If more police officers were sent to patrole the Westbank, then the IDF would be in more trouble, therefore Israel doesn't want too much police hanging around. The IDF is Lord and Master in the occupied territories and the Israeli state keeps it that way.
Man shot dead in northern Israel town of Bi'ina, police say the murder was gang related (Haaretz)
from the article: In Israel, the life of a Palestinian is cheap