you have done nothing wrong. When the blossom was out the weather was probably too cool/wet for bees etc. A mate has only 1 fruit on his and it usually has plenty. But we had frosts when the blossom was at its best. The frost kills the chance of fertilization sadly.

Very frustrating isn't it? My young cherry plum hasn't even flowered yet. Nothing to be done about it unfortunately...
As time goes I am hoping I live long enough to at least get a taste as I've never had or even seen a cherry plum. Try giving your tree some sulphate of potash.

Take your pick. As Eileen says, as pears tend to come into blossom early, then frosts can burn off the blossom. Some varieties need pollinators to make them fruit more reliably and some varieties produce fruit one year and barren the next year (Biennial fruiting).