Please, help me understand why

Recently, my 6 year online best friend cutted me off.

Their message mentioned they have been thinking of this for too long, that this friendship isn't doing them any good anymore. That their mental health hasn't been well and they needed a break.

Before that message, they've been gone for a month. It wasn't the first time they went away for a long time, tho it was never this long. I fully understand that they're busy, so was I, and I was willing to wait for whenever they can to just come by. In that month, from time to time consistently I've been messaging them, sharing things, and checking on them. It was like the usual things I've always done. Some of you may think that they might've been overwhelmed, but we've talked about this before and they have said for me to feel free messaging them, while I also said to not have pressure in replying.

Besides that instance, I just don't know why this is happening. They told me before that they were getting better and is seeing a therapist. So, why? If they had mentioned something else before all of this, like needing a break or how things were going for them recently, I would've understood or be there. We could've still talk about it, but instead, they sent that message.

I still want to stay close friends, though even if I try to salvage this, I know it will never be the same ; and they've made their decision. There's no going back. I feel incredibly hurt. For the time we've spent, I wish they could've at least spared me a proper goodbye. Instead, I am left with more questions than answers. I want to know what was it that pushed them to do this, but can I really ask? I have yet to reply to them.