People with young dc don't want their bedroom to be on a different level (children needing to go up/downstairs at night to get to parents).
A lot of the total floor space is wasted with stairwells and landings.
The communal space often closer in size to that of a 2 bed, even with 4 bed town houses
Or having things like a "living dining room" on the 1st floor but the kitchen downstairs, carrying food up and down seems a faff, so kitchens need to be big enough for a proper dining table, but as often only are if you give up the garage.
Additional bathrooms to make sure there's a full bathroom on every floor with a bedroom, whereas if that was a normal house layout, you'd cope with one family bathroom, loo downstairs then have the extra room/s as extra bedrooms/study/to make the bedrooms bigger.
Often have pokey gardens and not enough parking.
I actually like townhouses, but they do need to be big to make up for the layout issues.