I think NECA's size selections are probably based on figure prices, their customer base and what other companies make.
I think they committed to the T2 12" T-800s before knowing HT was stepping up and may not have if they'd known, they probably figured, their competition at the time was the Sideshow figures, their 12" T-800s would have some appeal, and they were right, I got the Pescadero 12" figuring it was the best chance to have a nice looking T-800 at the scale, but I probably wouldn't have gotten it if I'd known about the HT line, not that for what it is, the NECA 12" is no good, just no sense spending the redundant money, and I would still have all the 7" NECA figures. Likewise, they probably didn't explore 18" T2 because, whether you like it or not, Sideshow already tackled that with the PF, so there was a competitive product. Their 7 inch scale only has old McFarlane figures to compete with and by having likeness for Arnie, they already have an edge of McFarlane, plus doing multiple figures.
The 7 inch line is very affordable for the younger consumers that frequent shops in the U.S. where NECA is primarily sold, their size allows consumers to buy more and take up less space and it's easier and more cost effective to build a whole line. NECA's larger stuff has always been a figure here or there, but there's never been a line of them, 7 inch is the only scale to get numerous figures from a property.