The standard explanation is that it is structurally a better solution, which is true (marginally). Another one is that it saves a bit of money, which is even more marginal.
I think it simply dates back to the post-WW2 days when people were a) smaller and thinner and b) much less fussy about creature comforts.
Nobody would buy a 1-door plane today when they could have a 2-door one. The difference in "people-friendliness" is vast. No suprise that Cirrus and Lancair (Cessna) have gone for 2 doors.