Why it matters
It isn't obvious to everyone why a mission is important. But companies that understand why they are expending the effort have clear advantages. Let's assume that you are a company that creates products that help the disabled live more independently, and your mission is
to allow everyone to live as free and full a life as they desire.
Benefits of having your employees on board with your mission:
Inspired employees and teams overcome obstacles and meet deadlines better
We need to finish this project before the convention. We could get a lot of sales if we can demo there.
We need to finish this project before the convention. Think of all the people who will be there whose lives we could change!
Employees can make more decisions independently when they have guidelines to work from
People will converge and agree with decisions easier when they align with the mission
Option A would be fast and easy, but Option B would better help our customers
Employees feel fulfilled by the work they do
What to do
Knowing why your mission is important, how do you get everyone on board?
1. Use your mission in the hiring process
Along with meeting the technical and interpersonal skills, how well do your candidates fit with your mission? Do they feel inspired, passionate and excited when you discuss it with them? The easiest way to get employees to connect with your mission is to hire the ones who don't need your help doing so.
2. Overcommunicate your mission
It sounds like the only way you talk to your employees about you vision is through a newsletter which is largely ignored. If that's the case, of course it isn't important to your employees because it isn't important to the management. Your mission should come up everywhere. Meetings which are purely about mechanics of the job communicate that the mechanisms are most important. Meetings that state how project X or feature Y aligns with the mission "connect the dots" for them. When you do this enough, employees will start to do it, too.
Of course, for any of this to work...
3. Your mission must be worth talking about
Is your statement inspiring or boring? Specific or vague? Is it a worthy cause? Of course employees will be uninspired by boring missions, and fail to understand how their tasks relate to an ethereal statement ("Make the world a better place"). Ensure that your mission is something meaningful, and not a exercise meant to check a box.
Good luck, I wish you the best, and hope that your company finds some magic.