the reason is simple - there's a lot of space and capacity in that old airframe and modern kit is small. You can also cheerfully cut holes in them without a lot of worry
Boscombe and the RAE used to use a Fairey Surveys operated DC-3 G-ALWC up to 1982 for trials of things like Linescan. If you fitted tanks in the cabin you could get 8++ hours of flying at a time