Well, it's due to several reasons. To begin with, SUVs are pretty comfortable due to offering higher ride height and higher seating position – it is simply easier to get in (those with backaches especially appreciate it). In addition, SUVs offer a lot of space inside and are generally very practical – you can take a road trip in them, do mild off-road, haul stuff. Old SUVs had, of course, disadvantages in having lower speed, acceleration, worse handling, and fuel economy than saloon counterparts – but this is not the case anymore. So, in general, modern SUVs provide better value for an average buyer, which ensures stable demand.