I think redonihunter has summed things up pretty well!
One thing I would add as a reason why indie horror games appear to be popular in general is a side effect of the way AAA horror games are released. Those developers seem to have decided people are only interested in playing horror games around Halloween/October/Spooky Season and so they all target that period. If you want to find new horror games the rest of the year you have no choice but to look at indie titles. I can't think of any other genre that has its release dates targeted in a similar way. The same thing also applies to movies.