They have three obstacles to producing a nice full pot. In order to build up numbers, seed is the only viable option. Division takes too many years to build up numbers, and tissue culture hasn't been worked out yet for these plants. First of all, the seeds have to be sown fresh, or stored very carefully in moist packages for a short time. Next, many of these plants have tricky seed dormancy problems.
Trillium takes two cold treatments, interspersed with warm periods, before germinating.
Bloodroot, hepatica, asarum and Cornus canadensis all require one cold treatment. This all takes time, some finesse, and space in the nursery, as the plants have to sit around during these treatments in seed flats. From a grower's point of view, this has many implications. First, a short growing season means longer time in years to reaching flowering size. Also, these plants respond much better to transplanting if done while they are actively growing in the spring.