The speaker refers to Autumn as the "Season of mists and mellow fruitfulness" because he wishes to honor and compliment the season whose hallmarks some might see as less beautiful than "the songs of spring." On the contrary, this speaker feels that Autumn has its own "music" that is absolutely as lovely as Spring.

One aspect of fall that makes it so attractive is how ripe all of the harvests are. The vines are "load[ed] and bless[ed] / With fruit," and fat, juicy apples bend "the moss'ed cottage-trees." The season and the sun "swell the gourd, and plump the hazel shells / With a sweet kernel," and the multitude of "flowers for the bees" make them think that warm days will last forever. There is such a sense of abundance and plenty, and beauty in ripeness. Further, the "drows[y] . . . fume of poppies" and the "last oozings [of] hours" help to explain why all this "fruitfulness" feels so "mellow."

In the final stanza, the speaker references the "barred clouds [which] bloom [on] the soft-dying day, / And touch the stubble-plains with rosy hue." If you can imagine an early fall sunset with misty clouds overhead that help to paint the sky with rose and salmon tones, then Keats's reference to "mists" becomes a bit clearer too.

In 1819, the final year of productivity as a full-time poet, John Keats wrote five odes, one of which was the masterful "To Autumn". Arranged in three stanzas of 11 lines each, "To Autumn" pays tribute to the fecundity and fruition of the season. Stanza 1, with the first line, “Seasons of mists and mellow fruitfulness” is a paean to the early days of autumn when nature has fulfilled the promise of summer. The stanza nearly overwhelms the reader with the intensity of its description of ripeness. The apples trees bend with their load of fruit, while the gourd “swells” and the “hazel shells” “plump…with a sweet kernel”. In the overflowing honeycomb of the last four lines the poet implies a runaway fecundity. Indeed, the grammar itself of the Stanza 1 reinforces this theme. Intended to be read as one sentence, and scaffolded by parallel infinitives, the first stanza nevertheless lacks a verb. By this omission, the poet – by grammar and punctuation – intends to convey the promise of an endless “fruitfulness”.  

In Keats' "Ode to Autumn," why is autumn called the season of "mellow fruitfulness"?

In his ode "To Autumn", Keats describes the season in vivid terms as being full of "mists and mellow fruitfulness." This creates a rich sensory impression of autumn, characterizing it according to the misty, foggy mornings and evenings which often mark the transition between summer and winter, particularly in the United Kingdom. Indeed, autumn is a transitional period in general- mist and fog are often used to symbolize obfuscation or dreaminess, and here they seem to signal the languid turn of the seasons.

Fruitfulness, meanwhile, is another defining characteristic of autumn. "Mellow" is an interesting choice of word- it usually means easy, or friendly. For autumn to be fruitful in a mellow way, then, suggests that this fruitfulness comes easily to it. It is a rich season, abundant with harvest, a time in which it is easy to find apples falling from trees and crops ready to be picked and turned into food. Autumn is not a harsh season, like winter, when the earth is hard with frost and food is hard to find in the wild. Autumn being mellow and fruitful conjures images of it as a welcoming, pleasant period.