Canada needs to keep its limited supply of troops available, to support any U.S. calls for a "coalition of the willing". A yuge amount of Canadian foreign policy is to avoid annoying Trump, so keeping the ability to support the U.S. is far more important than the "prestige" of winning a U.N. Security Council seat. Canada is unlikely to get anywhere near the NATO military spending commitment, so being able to send troops is at least a decent "out".