The thing is, pirates now DO have places they can sell most loot for a good amount. Its just as profitable, just higher per-unit prices instead of a bunch of random stuff.
As for why people use gunboats or bombers? Simple, bombers can ream anything but fighters. Gunboats also are generally tougher than the traders they prey on. It really comes down to the pirates not wanting to actually have a challenge and expect to be handed goods or credits without a fight. And for a long time this was how it ws becuase all lawful transports dident have teeth. Now with the battletransports convoys are actually dangerous.