Post by Marco MoockPost by Rich"xset dpms force off" and the panel turns off.
That works, so DPMS itself is possible.
The questions is now why screensaver can't handle that.
If you mean "why does the screen not power down with xscreensaver" --
then the reason might just be that "screen savers" are a legacy from
the CRT days intended to prevent phosphor burn in by displaying a
moving/changing image when the system is idle (and predating the days
of DPMS powerdown control). They were originally meant to draw
animations, not simply power down the screen.
Do note that xscreensaver's man page claims it has support for
triggering powerdown, but you do have to configure it to do so (see the
section "Power Management" in the xscreensaver man page).
But, if the only reason you run xscreensaver is for idle power down,
then that is silly, as X already supports idle time-out power down
natively, so xscreensaver is not needed for that singlar purpose.
If, however, you are also using the "screen locking" feature of
xscreensaver, then you do need to run it to get the screen locking
feature -- and for powerdown you'll need to tell it to perform power
off as well (or configure X to power down a short time after the idle
lock engages).