It is a question of trust. Unless you are a proven writer or an expert in the field you are supposed to write about, you’ll just have to earn it. If you get a chance.
The problem is, you usually have to make a contact through an e-mail, social media message, or a writing of yours, some writing. Simply, based on this, your possible employer has to have a hunch that you will do a good job. Without any direct contact of some sort a seasoned employer is not bound to do that. To always remind you of this situation, you should have a video of your dog (or neighbour’s dog, doesn’t matter) chasing its tail playing regularly as your screensaver. It might inspire you to come up with something that will give your possible employers that hunch they need to take that perceived risk of theirs.
Simply, you just can’t stick only to your ‘expertise’. You might have your hands full at the moment, but what when the jobs in that field dry up, or competition (cheaper at that) ones up, or you simply get tired of writing about the ‘same old’? You have to try something else. Maybe you want to go back to that old office routine? I guess not.
Of course, giving up is not an option. As another reminder, you can from time to time play Leonard Cohen’s prophetic “The Future” - it can definitely be “murder”. So, relax and think what else is possible, how you can approach the suspicions that will be there, no matter what. Because, it is possible.