Because the HR20 is still not made by Directv, or designed as such like the d10-11 r-15, and h20 are... it is like when they switched to directv brand originally. They are still made by the tivo manufacturer and as such have a tivo power cord.
The new HR 20-700 has nothing to do with TIVO.
I'm not sure who made the HR10-250 TIVO unit (I don't remember) but it was TIVO branded.
The HR20-700 is made by PACE
As Charper said, none of the receiver are actually made by D*, but they have the rights to the design.
Jimbo