Because Mesosphere (the VC backed startup) hired the creator of Mesos (the Apache project), and is now trying to capitalize on the open source project as one of their "products".
Mesosphere is arguably the largest contributor to Mesos, certainly on par with Twitter, especially when you consider all the surrounding ecosystem. The company also secured permission from the Apache foundation with the trademark when the company was founded. It's good for the open source ecosystem to have companies productize and support projects, particularly when they are plowing millions of dollars back into the open source.