IMHO, It's the driveline. Two rear wheels makes for more non-standard bike parts. Less serviceable by LBS.

Another item would be less rolling resistance from 3 wheels than 4.

Also the tadpole trike configuration is quite able to stop quickly with just the 2 front brakes, even on a loaded tandem version.

To answer beelz question...
No, four wheels is not worth the trouble in a practical sense.

Although it has been done with the kit that connects 2 EZ-1 recumbents to the Crank-It fullsuspension quad off-roader. There are also lean steer trikes & quads.

All very cool, but few that are practical...AND cool.

That would be a Greenspeed!