Why don't using bidirectional ray tracing algorithm?

[LiuZengqiang]

As mentioned in Visual HFLCAL - A Software Tool for Layout and Optimisation of Heliostat Fields, the bidirectional ray tracing algorithms ,that generate ray on the surface of heliostats, will get better performance by getting rid of calculating the intersection of ray with heliostat and the rays casting to ground.
Why SolTrace don't using bidirectional ray tracing algorithms to accelerate ?
Look forward to your reply.