As P44 said, It is a cost thing. How much does the average punter want to spend on their plants?
Growing submersed has increased costs for several reasons. Requires more time, More light, More space and more labour.
Growing emersed is by far more cost effective. Thus meaning the retail costs are kept lower.
Converting the plants to submersed is affected by several factors.
Firstly the type of plant. Stem plants will convert much faster. Rosette plants rely on their roots for nutrients and they are slower growers. They need to establish their roots before they are able to grow new foliage.
Light is another. The more light the more the plant can grow. Many underestimate the light requirements for growing plants.
Another common factor is how the plants are planted. A lot of people take a bunch of nice plants and just ram it into the gravel and expect it to grow.
Damage occurs, the plant has no nutrients and the plant is restricted from light as it is in a clump.
These are just a few things.