The answer is simply, Moore's Law.
In its usual formulation, it's expressed as "the number of transistors on an economic-to-produce chip roughly doubles every 18 months".
However, looked at another way, it could also say that if the "capability" (i.e. number of transistors) is held fixed, then the cost can be driven down over time.
This is the view that the Java ME executive committee took, so the reference devices are no more powerful than they were 4 or more years ago - but they are much cheaper. This matters to the space that CDC is trying to play in, but it does mean that devices are typically underpowered compared to what's needed for an SE subset.
A rebaseline of the CDC spec is underway, to bring it closer to SE. Java 8, with its modularity support will also make this easier.
Ultimately the goal is convergence of ME to a subset of SE, but this will take several more releases yet.