Linkage forks have been tried on motorbikes too. Like every solution in engineering there are pro’s and con’s and I think the cons of linkage systems simply mean the overall net benefit is not there, apart from weight, cost and complexity, a downside to linkage setups are poor feedback, which is quite important on a mountain bike to give the rider confidence.
My experience on the Lefty was that the fork was noticeably stiffer and the performance/activity of the suspension under braking – which I’d never really felt before as stiction increases with conventional bushed forks so they don’t work as well under braking. This gave me a lot more confidence under braking on choppy surfaces. Something I miss with my current Pikes.
However the Lefty did suffer a lot more braking dive than a conventional fork because of the lack of binding up of the fork action. OK you can easily compensate once you’re used to it by moving your weight rearwards when braking to use weight to counteract it, but then you’re unweighting your front end at a time you want to weight it up.
So on balance there were overall benefits to the lefty, but they do have their disadvantages too.