It is because running the speakers parallel cuts the Ohms to 2. Ohms is a measure of resistance so the lower the number the lower the resistance. If the amp isn’t stable at that load, you would not be able to wire it in this configuration without damaging it. In order to run a mono load on an amp only stable at 4 ohms, you could either use a single 4 ohm speaker or two 2 ohm speakers in series to achieve a 4 ohm load.
Parallel speakers means the + from the amp hooks to both + terminals on the speakers. Same for the - terminals.
Running speakers in series means the + from the amp connects to the + of the first speaker, the - terminal from the first speaker connects to the + terminal on the second speaker and the - terminal on the second speaker connects to the - output of the amp.
The bridging of channels is done electronically to give you more options with the amp, but ultimately is dependent on how you are wiring it up to the output.
Hopefully that answers some questions for you.
It does. Thank you!