How come the RPMs are not placed in alphabetical order throughout CDs 2~6 ???

For example, eclipse-related RPMs are dispersed in several CDs.

Wouldn't it be better to have RPMs in CDs in such a way I mentioned above, or is there any specific or interesting reason behind it?

They're placed in order of needfulness. Not everyone likes downloading 6 CDs.
Doing it alphabetically would be a mess during installation.
No one likes swapping CDs every 30 seconds

Eclipse is actually a smaller package, but it needs other packages called "dependencies", in order to function. These "dependencies" are sometimes required by other types of software and may already be installed if they were a dependency for something else you installed. But because they were not installed prior to eclipse, they now need to be installed. They are not necessarily a 'eclipse-specific' package, rather a separate type of software used by many applications (common files in Windows).
If you would rather have all these dependencies grouped up and installed with any application that requires them every time you install a different application, install Windows. Then you can have things like Adobe reader 4, 5, 6, 7, & 8 all installed at the same time when only one version is required to display .pdf documents. Linux does not install multiple versions of the same dependency. That's the difference, and reason, packages "seem", to not be in order.