I thing a school of thought developed that such planes with machine gun
defensive armament still could not economically operate in an
environment with cannon armed enemy aircraft present without fighter
protection and that the weight of gunners, radiomen, and navigators were
not worth the loss in performance/payload....of course, events might
have proven that rationale wrong and like the Il-2, a decision might
have been made to add a crewman and defensive armament. Just because
the decision was made didn't make it correct. Under actual combat
conditions a .5 cal turret still might have been quite useful.

Of course, technology also helped get rid of the navigator/radioman for
some applications.

Look out the Air Force and Navy have bounced around with single and
muliseat figther and attack aircraft for various reasons.

KDR wrote:

Just wondering how come post-WW2 carrierborne torpedo planes such as
Firebrand, AM Mauler, Wyvern were all single-seaters. Only a few years
ago during the WW2, carrierborne torpedo planes required three crews -
pilot, bomber/navigator & radioman/gunner - without exception. What
brought this change?