Why can't we have large regions anymore?

Gen 1: 25 routes
Gen 2: 23 routes (48 counting Kanto)
Gen 3: 34 routes
Gen 4: 30 routes
Gen 5: 18 routes (23 in BW2)
Gen 6: 22 routes
Gen 7: 17 routes (24 counting outdoor Poni Island areas)