The first question is whether you really want to write. Not like ‘all else being equal’, like if time was free. But given the actual costs and opportunity costs.
If that answer is yes, then here’s the case for writing in the morning.
You cannot guarantee what will happen after you leave the house. After you start checking email. After work. After you get ready for bed.
But first thing in the morning, that’s your time.
Growing skill and achieving success is about practice. And for something like writing, getting practice is about building the habit. Only with habit will you do it frequently enough, over a long enough time, that you’ll build the skill.
That’s the number one thing for writing.
If you want to write, if it’s true that writing is the most important thing, the thing you want to do, then do it first. Do it when you wake up in the morning, or as soon as humanly possible thereafter.
Why not write in the evenings? Because at that point, it’s too easy to push it back. Until eventually, the trade off you’re making is between sleep and writing. That’s not a place you want to get yourself.
Why not schedule time during the day? Because stuff will come up.
Also, your energy is highest in the morning.
Minimize the number of variable items between the moment you wake up and writing. You can do routine, and habitual things where writing becomes one in the chain. But minimize the variable things. Don’t check your email first. Don’t do a phone call. Try not to see people, if you can prevent it. They’re unpredictable.