Because our defence just isn't that good. Hill has tried many different combinations in defence and none of them have worked that well.
Maybe its because we play open entaintaining football that you have to except we will leak goals but hopefully score more than the opposition.
Has he changed the back four that often? Other than when Foster was out and Edwards quite rightly got two games in a row after his West Ham performance, it's pretty much been the same back four for the last seven/eight weeks hasn't it?