I voted on the “MARKETING REASONS” but the truth its not as simple as many of you think. 
The real truth are more complex then these poll alternatives. I have both close friends that work as developments, tech-nods, full time professional programmers, game manufactures and these kind of companies is a large boat to navigate. Its not as simple that they don’t want to add all the features. There are tons of meetings “in-house”, fixes and structures that need to be done and are up on “the daily agenda”, before just have the time to think of doing anything else.
And even if you do, its not as simple as just grab a computer at the company and start program some codes and add it to Cubase 7 right away.
Someone else, “teams” working perhaps on other features that need to be approved and “co-sync” so it work with your new code before your new code get implemented to the real system.
So as an example, there are probably more then 1000 of lines of codes that need to be program just to make a simple fix of example “Plugins windows” stay on top, not fall behind the mixer-page in full mode. Its very complex thing to do and they will do it but it will take some time to fix it. In the same time they need to work on fixes to 7.5 they also need to continue program all the new things that will come in Cubase 8.
Think about it!
Best Regards
Freddie