At the risk of being burned as a heretic, I see no empirical reason viz. a fundamental difference in their shoes: Whether it's a $150 pair of Indian-made Loakes, or a $1500 pair of John Lobbs, the fundamental nature of a goodyear-welted shoe is the same: Glued insole, machine stitched welt and sole. The differences are in leather quality, attention to detail/finishing and such like, and then the purely subjective issues of fit and design.
Even in the quality issues, once you get past the $300-500 range, the returns of spending more are going to diminish rapidly. Is a $500 pair of Crockett and Jones shoes better than a $250 pair of Barker shoes? A bit. But the Barkers are a lot better than the $100 Loakes and the Loakes are a hundred times better than a glued horror from Aldo. And at the other end, once you go from the $500 C&Js to the $1000 Edward Green, the differences are very small indeed in my opinion, and the added value per dollar almost negligible. Factor in the availability of many other non-English quality makers for lower prices, and even the availability of hand-welted shoes for under $500, and one wonders why Green, Lobb or G&G are able to sell shoes at all.
The reason they are is that those small differences still matter to some, and just having the "prestige" brands also matters to the less self-assured. The only quantum leap to be taken in quality is to go to the $5000+ gentlemen's crack addiction of fine bespoke. But if you're buying RTW or MTO, then the only answer do your initial question should be this: "Because they fit really well, and you love them." In that order, I would suggest.