Because the OS you use will access the internal from time to time and slow down the read/write speed of the editing process if you use the internal to also store video you edit on.
And as editing wears down the HDD much faster than the OS does, using the internal HDD as scratch disk could also lead to a faster deterioration of the internal HDD.
If you have a Mac with the ability to have more than one HDD internally (PowerMac or Mac Pro or even MB/P via OptiBay) you can also use another internal HDD as scratch disk.