Two reasons: 1. In general the large oil & gas companies have been out of favor with really poor earnings growth. 2. OXY in particular was run as an individual fiefdom by the former Chairman, with little reason for most of what they did. They owned way too much property, much of which would take years to develop and on top of that they were poor operators, wasting lots of CAPEX in drilling.
OXY is now in the restructuring mode, but it is not easy to undo years of mismanagement. I think the institutional investors are waiting to see what the outcome of the restructuring will be.