Jon marrying Shireen would deny them the possibility of a northern Queen and a marriage to a dragonrider at the same time. He knew that Wyman also wanted to make his granddaughter queen, but the Lord of White Harbour seemed to be afraid of a second house having dragons in the North. Or was it maybe because the main line of House Manderly had no male in a marriageable age and he certainly knew that? It seemed that his Lord Hand was thinking about the long-term consequences and preferred stability. No doubt if Wylla couldn't become queen, a future great-granddaughter of his could be married into House Stark in the next generation instead.