MiKTeX is an implementation of TeX and related programs for Windows (according to miktex.org). Therefore its main goal is integration of TeX and related programs, including its package manager, into Windows operating system.
TeXLive is designed to be a cross platform distribution. Its package manager must work in a number of operating systems, and must integrate to some extent with all of them.
The goals of both distributions are somewhat different, which explains why their authors decided to take their own approach to variety of problems, including package management. While it is possible that both distributions, or at least their package managers, will converge together in the future, it is not necessary for that to happen, and it is not even clear to me that it would be a desirable thing to happen. It may reduce work, on the other hand, it would reduce users options, and possibly lead to a slowdown in innovation.
Also, while implementing such common package manager may reduce work in the long run, it would certainly increase the amount of work significantly in a short run.
A good read for different aspects of these distributions: What are the advantages of TeX Live over MiKTeX?