A Very simply, your strawberry tree (Arbutus unedo) failed to establish a good root system. It should have easily doubled its root spread over 18 months, but it appears to have made very few, if any, new roots. Gardeners often contribute to establishment failure by poor planting technique. The main failure is through not thoroughly teasing out the rootball prior to planting. The all-too-often-heard advice of not disturbing roots is just nonsense.
Container-grown plants are well-grown at nurseries, but invariably root bound. It’s essential to thoroughly tease out the rootball to get these roots growing out into the surrounding soil, which is very different to the potting compost they’re grown in. You can’t usually do that with your fingers. I use a patio weeding tool, but a sharpened piece of bamboo, or similar, will do the job. Long, circling roots should be uncurled and pruned off at the side of the rootball.
Make the planting hole no deeper than the depth of the rootball, but three times as wide, as roots tend to grow sideways, not down.
Finally, don’t add organic matter to the planting hole as this tends to discourage rooting out into the surrounding soil. On very poor sandy soils you might incorporate some as part of the backfill (the soil dug from the hole).
Secure the tree trunk a third of the way up a tree stake, inserted diagonally at 45 degrees. This will ensure the new roots aren’t broken if the tree’s subject to high winds.
Planting too deep is the other major contributing factor. Again, the advice to plant at the depth the tree was in the pot is misguided as a lot of nursery stock is mechanically potted and is too deep to start with. This is the case with your tree, which was potted at least 5cm (2in), if not 7.5cm (3in), too deep. The tree has produced additional adventitious roots from the buried trunk to try to compensate but without success. With poor root growth, the tree has eventually come to a halt and died.