Probably too much rain..get rid of them and try sour cherries.

If you get rid of Stella, what will cross pollinate the Rainier? Unless you plan on planting another variety to pollinate Rainier. Rainier is not self pollinating. It sounds like both your sweet cherries should be removed.

I had bacterial canker on one of my cherries. I tried to prune it out which was a lost cause but did allow me to keep the tree a few more years until the canker spread so badly I gave up and removed the tree. This was on a tart cherry too!