I'm not sure but I'm guessing there are two main reasons: people still running the old protocol or nodes who cannot share slices from the account tree for one reason or another. Many of the exchange nodes probably have slice sharing disabled because it can freeze up while generating slices. But I've never really had much trouble syncing until recently so it could be something else.

There is an alternative to the Qt wallet which provides more advanced coin control but it's a bit complicated to set up: https://github.com/MiniblockchainProject/WebWalletQtAlt I will probably release a packaged version which is easy to install and run but I want to fix a few bugs I found the other day first. The problem is I'm just so busy working on other stuff at the moment I don't have much time to focus on Cryptonite. The biggest mistake I made with Cryptonite was probably not doing a pre-mine to fund future development. I figured a pre-mine wasn't necessary because I already had funding for the initial development of Cryptonite and I was really expecting more developers to come in and help work on Cryptonite as an open source project once we released the code but it doesn't look like that will happen.