I suspect you generalise too much from one family -
take 1750-1799 - 11 out of 521 Maughold marriages noted a party from Braddan
in Braddan only 7 out of 1548 marriages in same period noted a party from Maughold.

Douglas was a magnet for many pre 1765 and tho revestment slowed things by about 20 yrs later it seemed to have recovered and again started to grow. Ramsey tho having a good bay did not have a good harbour - thus many families from throughout the Island would have links to Douglas and thus easy access to possible marriage partners drawn from second cousins etc