First of all, 13 mini is still on sale, not a discontinued product. It will be probably be on sale even after the iPhone 15 comes as well.
Second, iPhone 14 has the same SoC and cameras introduced by the 13, and a 14 mini would have been incredibly redundant as an upgrade, it made more sense for them to explore a new type of product with the 14 plus.
Third factor, Apple now has the time to gauge interest for the 14plus, which looks to be quite scarce to be honest. People mostly buy the middle form factor anyway. Apple could probably sell only an iPhone SE (refreshed, XR based), the 14, and the 14 Pro / Pro Max, and people would likely find their right product anyway. With 4 phones instead of the 8 they sell today.
Bottom line, The 14plus still has more chance to survive because it's an upsell from the 14 and makes more money for them, whereas the 13mini was a downsell from the 13 and made slimmer profits for them. That's really the most important point, they care more about that than about the screen size.