George Lucas not completely satisfied
From what I've read on other internet sites (can not remember what they are, it's been a while), there are certain things about this "older" Clone Wars series that George Lucas did not like or wanted to change. Therefore he decided to not only create the "newer" series to continue telling the Clone Wars saga, but is also using it as a way to correct a few things from the "older" series. Which is also why the DVD's for this series were discontinued by Lucasarts. Apparently some things are no longer "canon" from this series.
Having said that, the sites I retrieved this information from never did go into any detail about what was changed between the two series', or why the DVD's themselves were taken off the market. Just that George Lucas himself wasn't completely satisfied with this "older" series. Now, everything I have just explained is merely a rumor at this point. But a rumor based on sound evidence. However, if anybody can verify any of this, and give specific examples of what GL did not like about this Clone Wars and what is being changed in the new series, I would greatly appreciate it if you could give us some much needed insight on this subject.