Because no art historian has come up with a better term.
They did not really have much in common, besides working in the decades after the breakthrough of impressionism. The artists who are usually called postimpressionists are Gauguin, Cézanne, Seurat and van Gogh.
All the impressionists.
The Impressionists
Impressionists use Canvas, Paintbrush, Paint, Water.
Among the famous impressionists only Renoir worked in Provence. There were other famous painters in Provence, but they were not impressionists: van Gogh, Gauguin, Cézanne, Bonnard, Signac.
No artists wanted to be in a group with van Gogh, Paul Gauguin tried in vain. These two are usually called post-impressionists.
The impressionists: Monet, Degas, Pissarro, Renoir, Sisley, and Morisot.
Edgar Degas, and to some extent the other impressionists. Also Japanese prints.
He was friends with the whole gang of impressionists. He always worked on his own, though.
All the Impressionists, Paul Gauguin, Henri de Toulouse-Lautrec, just to mention some of the French artists.
Claude Monet and Auguste Renoir were impressionists. Édouard Manet belonged to the same circle of friends and colleagues, but only for a short period painted in an impressionist way.
He dreamed of collecting a group artists at Arles around Paul Gauguin and himself, but it did not happen. Nowadays he and Gauguin are labeled Post-Impressionists.