Dependable schedules are one reason, the other big reason is that sails interfere with loading and unloading the boat.
Modern shipping extensively uses cargo containers that are rapidly loaded and unloaded using cranes. This advance has drastically lowered the per-unit costs of shipping freight in the last half-century (check out the book "The Box" for more details).
If adding sails makes it difficult to use a crane to unload containers from the deck of a boat (likely, imo), then it would make the per-unit cost of shipping skyrocket.
"Conversion, fastidious Goddess, loves blood better than brick, and feasts most subtly on the human will." -- Virginia Woolf, "Mrs. Dalloway"