I believe this is to make people aware of that someone they have voted for are no longer running or have issues with their witness node. If they where not visible they wouldn't be able to remove their vote.

However, there could be a list of "Disabled Witnesses" if you had a vote for someone that was disabled. Just like you now have a list of "Not Top 100 Witnesses".

Though, none should have a vote for someone that has a disabled witness cause it hurts the network in so many ways. Not only aren't they adding towards the support they have been given, but they are hindering new witnesses from gaining their share of the votes as well.

Sure, it's up to every witness to promote themselves as well, but it could all be much more clear.

I'm also a witness, and recently just entered the new Top 100 list, making me more visible. Though, if some of the disabled witnesses where not listed in this list I would have been on it way longer as I'm currently the 88th Active Witness. Source: https://steemian.info/witnesses.