Perhaps not consciously but what other reason could there be? In the end it comes to the thought "Can I trust that in the end, there are other people with enough SP and good morals to make sure Steem will have good top witnesses"
Either one has that trust in itself or they don't. Perhaps decreasing witness voting slots would encourage current high SP holders to actually care who they give Steem via upvoting rather than just selling them for the highest bidders...
Steem is a blockchain, decentralization is something we should strive for. One account having enough power to decide who gets to be top witness and who not is something that will drive people away from the platform, the ones with serious money, who we sorely need here to drive adoption. Those people look at the fundamentals.
I'm still in favor of limiting witness voting spots, 7 could be a bit extreme, even though I'd want to see that happen, yet I'd settle for 10.