Why would one want to exclude them? Maybe one of them has a pretty good idea because the environment makes him/her think a lot about it, for years.
I know it may be due to some rules which are typical when there's some prize to win like when collecting some supermarket points or whatever, but in this case? There's more to win for ESA than just a good design.
Because this is outreach. The goal is not to get space suit designs. The goal is to put the name and existence of the organisation into the head of as many people as possible.
To avoid collusion, or the perception of collusion.
You already paid for the ideas of your own employees. You don't want to pay twice or have the appearance of paying twice.
You don't do that by asking people for spacesuit designs, you do it by landing on comets.
Your ideas will be evaluated according to the follow evaluation criteria:
Representation of the European identity - both visually and within the accompanying text
Creative but realistic
Quality of the image
Innovative, inspiring design