The project had reportedly already integrated the major improvements from Servo into the main engine. It's not outlandish to assume that the marginal benefit of working on Servo was lower, compared to working on features on the main browser.

If you really want to go that route, you could make an argument that maybe Mozilla is handicapping Firefox's bookmark feature to get people to subscribe to Pocket... but people in this thread who are using Pocket also complain it doesn't solve their problems. What's the angle there? Pocket doesn't want paying customers?