Like someone else said, it’s because it’s a Southwest flight… Southwest isn't shown on the results of the big airfare search engines (kayak, expedia, orbitz) when searching from the United States because it refuses to pay those websites' fees.

Looks like you’ve found out that they are shown on Canada based version of the Kayak site. Most likely because the fee arrangements are different or because Southwest is willing to pay them, probably to get exposure to markets that wouldn't otherwise know about them because they don't have a marketing presence there.