Many of the Hive witnesses are more technical-oriented than marketing-oriented, so I don't think makes sense for all of the Hive witnesses to be involved in marketing (at least no more than anyone else involved in Hive).
I think there's a common misconception that Hive witnesses are the leaders of Hive, but this isn't the case. Witnesses serve two functions: producing blocks and voting on protocol upgrades.

Many witnesses do lead projects in Hive, but leading a project in Hive and being a Hive witness are completely separate things, although it isn't uncommon for people to decide to do both things, and maybe this is what leads to some of the confusion on the topic.

Realistically, if you want to increase the awareness of Hive via marketing, the best thing would be if more individual users decided to recruit their friends.