Largely because none of them are quite right, but we need somewhere to live and can't hang about for too much longer.
Our ideal would be to have a spare bedroom, large garden and walking distance to school (kids are between 4 and 8). This apparently doesn't exist. 
So ok, house 1 is brand new, loads of living space, spare bedroom but tiny garden. Walking distance to school and shops etc.
House 2 is smaller, no spare bedroom but masses of outside space. Fantastic garden. Converted coach house which needs a bit of updating (new kitchen/bathroom etc) but largely in good nick. Out in the middle of nowhere, would be a (short) drive to school.
House 3 also has no spare bedroom but decent size garden and walking distance to school. A lot cheaper than the other two, again would need some work, kitchen/bathroom etc.
Something else to consider is that we only expect to be here for a few years, so will need to resell within the next 5 years - this makes me think the new one would not be as good a bet.