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.
So you think that money is the root of all evil. Have you ever asked what is the root of money? -- Ayn Rand