Why do we associate turtles with turtle soup? | AskHistorians Archive Observer

Turtle soup used to be extremely popular from the 18th Century to the Victorian era. According to Food Timeline:

Turtle meat was highly prized by English diners from the 18th century forward. This exotic meat was the focal point of for wealthy feasts. Preparations were complicated; presentations were exquisite. Early recipes were served in the orginal shell. Mock turtle soup, substituting calve's heads for the title meat, surface shortly thereafter. Advertised as tasting like the "real thing," mock versions were readily consumed by middle class persons on both sides of the pond. Canned turtle soups (regular & mock) were introduced in the last quarter of the 19th century.

According to Soup Through the Ages: A Culinary History with Period Recipes:

Perhaps the most appreciated soup was turtle. The green sea turtle, so named because of its greenish fat, has been consumed for several hundred years and was once a major source of fresh food for exploring Europeans and pirates. Pliny write about cave-dwellers who ate turtle flesh although they worshipped the turtle. Throughout the 1600s and beyond, seafarers caught he great turtles and kept them on ships until they were killed and cooked...Turtle soup's virtue was that it did not 'cloy.' or produce ill effects, no matter the quantity eaten, even though it was often richly spiced...Isabella Beeton later pronounced turtle soup 'the most expensive soup brought to the table,' and advised that when live turtle was too dear [costly], many cooks used tinned turtle meat."

In Alice in Wonderland, the creature Mock Turtle was a joke based on cheaper substitute for real turtle soup.