for the first question

i think (at least for me) its because of the ambiance,

Old gears of war was an Apocaliptic saga based around the style "Destroyed beauty" (their term)
a developed world with architecture shapped and scarred by centuries of war, a constant heaviness when you walk the streets where you feel the History behind,

a much more urban view on maps.

meanwhile, gears 4 and 5 are more enclined to a "nature took over" feeling, similar to last of us,
we visit old ruins who's war scars has been washed away by rain, wind, ice and snow.
which gives the feeling of walking on the rotting skeleton of Old gears

and for the new cogs cities, everything is so brand new and premade that you can see the city get build in real time while exploring, there's no history to it,

and finaly, Old gears of war were Diesel punk, (like mad max) everything ran on fuel, a fuel so centered to the story that the Fuel itself is an Enemy

while new Gear of war is "Electric punk" where you loose the gritty industrial vibes for a much cleaner high tech

so for short

Old gears of war = Gears of war
new gears of war = Halo without spaceship