The size of the fuel tanks is probably one of the reasons. A Vanguard will not get much more than 1600 miles out of its tanks, while the Nimrod went up to 5700 miles. Installing extra tanks in the fuselage would eat into the space needed for sonobuoys, sensors and other electronics, so not really a solution. In comparison, the Lockheed Electra could do 2700 miles if not fully loaded. When they developed this into the P-3, it could do 5500 miles, which is significantly more than what the Vanguard had to offer.