On a tank type toilet they use the atmospheric pressure from the water in the tank to create the flushing action but a toilet without the tank needs the volume and pressure from a larger pipe to do the same thing. A toilet flush valve is always 1" and sometimes if the pressure is to low it still can have problems flushing. Also the flushvalve needs a copper pipe coming out of the wall that is supported so it cannot move so the fitting to connect it can be soldered on and you don't want it to be able to move around or that will cause leaks where the pipe goes into the bowl.