Why might beautiful looking morello cherries be tasteless?

Same as most supermarket food - forced, greenhoused, picked before ripe, kept cold until it arrives at the supermarket, often in an ethylene environment which causes some plants to actually ripen. These are known as climacteric.
See Wikipedia - Ripening

Cherries are actually non-climacteric, which means they only ripen whilst still on the plant. This means the time from picking to selling needs to be kept very short; if they're picked once fully ripe their shelf-life is hopelessly short for the supermarkets to sell through.

Also responsible may be big, fat, fast-growing strains of the product. Look good at the expense of flavour.

The solution… find a local greengrocer who actually cares, or at least pay the premium for the 'supermarket posh name' [Finest, Taste the Difference, etc] or organic [though that's still no guarantee].
My own personal gripe is supermarket coriander [cilantro to our transpondian friends] which from the supermarket is … just green stuff. From my local Turkish store, who import it themselves, you can smell it from next door. Chalk & cheese.

^{I remember, as a kid being taken to Blackpool for a weekend on the beach [50 years ago] you could smell the tomato greenhouses from the main road, a hundred yards away. The ones you bought in the town were picked yesterday. These days, that's why they sell 'on the vine' tomatoes - because the vine itself holds that smell long after the tomato has lost it.}