Why the difference? Because they are completely different types of entities in different businesses.
CHKM is a midstream/G&P MLP with a strong balance sheet and little commodity price exposure and with good growth prospects (from dropdowns from its parent).
CHKR is a royalty trust (declining asset) with significant exposure to natural gas and oil prices.
Bottom line: CHKR is higher risk than CHKM and therefore priced for a higher yield. Having said that, I expect CHKR will do OK and the price will gradually appreciate, especially once distributions start.