ipad mini is a great size, not everyone cares about PPI infact, why would anyone buy a surface if they did as it has a lower PPI than the mini.
people buy airs instead of retina pro's and but 21.5" mac apposed to 27".
a few reasons. might be they are lighter, smaller but still perfectly capable devices.
yes the difference is noticeable between the ipad 4 and the mini but really if thats something you worry about then get yourself a life.
i personally bought a mini and sold off a macbook to fund it. yes they are different devices but the mini is perfect for my needs.