A matter of history ..
(a) in the olden days, if it got off the ground, you gave thanks to the Almighty on a case by case basis
(b) progressively the Regulators tightened things up as the technical capabilities of design and manufacture improved. I recall doing a course more than a few years ago in which one of the guest lecturers was then an elderly chap who, as a young lad, had been a very junior engineer in the early US Regulatory Authority (Dept of Commerce was launched in the mid 20s ... became the Bureau of Air Commerce in the 30s .. became the Civil Aeronautics Authority in the late 30s ... the Civil Aeronautics Board in the early 40s ... the Federal Aviation Agency in the late 50s ... and then the Federal Aviation Administration). Some of the tales he told of finger in the wind early rule making (engineering judgements as we are wont to say) made for a very interesting yarn.
(c) eventually we ended up with two distinct categories of aeroplanes .. lighties (Austers, Tigers, etc) and heavy iron (DC3 etc). As there was a fairly big weight gap between the two, a finger in the wind separating point of 12500 lb was defined.
(d) progressively the design capabilities in the heavy end of the market saw increasing levels of redundancy which came at an increasing hull cost.
(e) while the seat mile costs might not have been horrific, the hull cost was for thin and small markets, especially developmental styles of markets
(f) hence, we saw the larger aircraft developing the scheduled services and the small fleet looking after the small end of the market. The reality of cost drivers meant that the Industry accepted lower levels of redundancy (and higher levels of risk = lesser "safety") at the low end of the market .. the alternative being no aviation services ...
(g) progressively, some small aircraft started to push at the 12,500 lb barrier and we saw the introduction of modified Standards, such as SFAR 41 and 23 which basically were a horse trading exercise along the lines of "we'll let you put in some more seats or go to a higher gross without requiring you to meet FAR 25 but we're going to impose some FAR 25 style requirements on you so that the travelling public gets a bit more for their trip ..."
(h) the question related to OEI performance .. you end up with a graduated range of capability from FAR 23 (little or none) through the Special Regs (bits and pieces as a sop to conscience) through FAR 25 (a set of modest capabilities). The discerning passenger will prefer a FAR 25 hull operation if he/she can get it .. ranging down to FAR 23 (or less) if that's all that's available ... I can recall a few trips out in the backblocks of Indonesia which raised my eyebrows from time to time as a consequence of this truism ....