Front End
Most bike engined and rear engined car trikes stick with telescopic forks as they do not have to support much weight.
  Most front mounted car engined trikes are built with leading-link(actually trailing link, if your gonna be picky) forks, as no matter how strong the telescopic forks are they are not designed for that particular use.

Before I have a screaming mob banging on my door I will just add that there are big car-engined trikes out there with telescopic forks fitted.  They usually have thick oil and heavy duty springs inside and out.
Heavy duty, V8 trike with the equivilent heavy duty springer forks.       Picture courtesy of Stryker trykes.
Theory

When braking with telescopic forks the suspension moment and braking moment act on the fork legs, causing them to flex, no matter how strong they are, even up-side-downies.

Leading link forks, as they have a swingarm suspended by shock absorbers are not affected in the same way as telescopic forks, as the braking and suspension moments are separated.

A bonus is that the leading links work as anti-dive as well.  They also come as single-sided, as in the case of reliant front ends.
suspension moment
T = Trails
Flexing
Trail no more than 1 1//2 in
Breaking moment
Front shock mounting.
CBR special.
Well-engineered single sided front end.
CBR special.