Some key residents of El Hierro did not want the necessary reservoirs for the pumped hydro storage to be constructed. The engineering is not that difficult but "politics" can be intractable.

Where there is both the will and a hill pumped hydro storage can work.

https://cleantechnica.com/2017/02/28/pumped-hydro-storage-secure-100-renewable-electricity-australia/

http://www.anu.edu.au/news/all-news/hydro-storage-can-secure-100-renewable-electricity