Why are thrust reversers not used to slow down to taxi speeds?
A minimum max reverse power speed is often an airplane operating limitation.  It's mostly related to FOD (mostly sand grains and small gravel) and on some designs there may be compressor stall issues due to flow disruptions. 

On the CRJ 700 max reverse is limited to 75kt although you can use up to 60% N1 down to zero speed.  On a 900 you have to be at idle below 60kt.

In practice, you start to come off max reverse during the landing roll transition from high energy to low energy (on the RJs, there is a speed call at 90 kts for this).  You come down to idle not too quickly and linger at idle for a second before stowing them; if you just slam the levers down to stow position, the reversers stow while the fan is still producing a lot of energy and you actually get a little kick of acceleration, which feels like you let off the brakes for a moment.

You can use idle reverse, which gives a little bit of braking, while taxiing if the alternative is riding the brakes and overheating them, say like a taxi along a downhill slope or along a slick icy surface where braking is marginal, and you don't want to taxi single engine, and in a pinch you may use reversers to try you save your butt if you start sliding off an icy taxiway with no braking control.