Why wouldn't I want to do this?

Possible reasons would include having enough intermod/bleed/noise on your
scanner as it is BEFORE adding the amplifier, in which case the amp would only
make it a lot worse. Also, if you have some noise but not much, but the amp
will boost signals below 30 MHz, you may find some added interference from
signals in that region.
IF it boosts FM freq's as well, you have that to deal with.
That said, though, if you already have a fairly clean system and just want a
bit of a kicker in receiving fringe stations, etc., there's no reason it
shouldn't improve things overall.
Just make sure you place it AT the antenna, not at the scanner... you want to
boost as much signal as you can BEFORE the noise added along the feedline, etc.
Good luck-
Linus