You forgot to add the parameters to your report.
Add rsExec.SetExecutionParameters(_parameters, string.empty); after the LoadReport.
_parameters = rs.GetReportParameters(_reportName, _historyID, _forRendering, _values, _credentials);
RE2005.ExecutionInfo ei = rsExec.LoadReport(_reportName, historyID);
rsExec.SetExecutionParameters(_parameters, string.empty);