Cap rate does not include repayment of loans (such as mortgage), and therefore does not represent reliably the annual balance. Why use cap ratio if another benchmark, such as ROI, provides similar information, but includes repayment of loans? There are many situations where ROI is negative and cap rate is positive, hence misleading.
In addition, using ROI (or rather, compound interest rate) as the benchmark, lets the investor compare RE investment vs. the stock market.