In general, it's the time issue.
They are fine if you wash one load a day, and set it in the morning before heading out to work.
Any other setup, and you would have to imagine the setup as follows: If that would be 2 seperate appliances, its as if you could ever only run either the washer or the dryer at one time, never both at once. Normal setups save huge amounts of time by basicly handeling 2 loads at once.
Further, space constraints. You could only ever wash half loads as you need space for the laundry to tumble during drying ( 1 to 2 ratio between washer and dryer).
Dryer drums never move side to side, thus, clearance between drum and cabinet can be as low as belt thickness plus 2mm.
Washers usually have clearances in the cm range due to suspension movement.
Further, there is basicly no way to fit a full size blower with well enough optimized air ducts and a proper sized heater.
Heating power has to be adapted to air movement capacitys of your blower.
Air intakes and outlets could ever only be placed above the water line.
And last but not least: Usually, wherever you have a 240V outlet which you need if you want anything more than 1.5kW of heating power, there is enough space to have seperate appliances.
On the situation in Europe: At least in Germany, they are verry rare.
You only find them in small condos or such, or holiday homes, barely as daily drivers.
Reason being that in most cases, we can just stack our compacts.
240V is standard anyway, so no power constraints.
Further, most use water condensing systems. Water here is beyond expensive, even more then electricity.
And, last but not least, even with our 240V standard, while drying, most only use 1.5kW anyway, simply because condensing systems can't handle more.
Temperatures have to be way higher to compensate for reduced condensing systems.
Oh, and venting is basicly obsolete around here. Heatpump dryers are so cheap now it really dosen't make any sense any more not to buy a heatpump dryer.
