I have chosen 43 and here are my considerations.
Glock's main selling point, in my opinion, is bullet-proof reliability and durability. And I want to believe, that all their designs are made to forster those:
1) Plastic cover ensures sufficient amortisation and flexibility for magazine not to be damaged when droppped or deformed when seating on. Wondering if that would be the case with thin metal magazines;
2) Not overcompressed, strong springs ensure long term durability (in fact, they even increased number of coils in 02 mags);
3) in general, sig patent "The magazine tube defines a single-stack portion extending to an upper tube end with an upper tube opening. The magazine tube defines a double-stack portion positioned below the single-stack portion and extending to a bottom tube end with a bottom tube opening." really seems like a b#ll****, since similar mags have been done by russians for a long time (see 12 round makarov magazines), and they did have a reliability issues.
4) thin magwell walls - again, for durability. I personally have dropped pistols about 5 times during ~10 years of carrying. Wouldn't like that the handle just crqcks in similar situation.
I dont believe that such a big company as Glock did not have engineering capability to stack 4 more rounds in 43, they just coundn't wo compromising reliability and hence reputation. Others just didn't have what to loose, and no one would by 6-7 round hellcat or p365, so they had to do smtg