Another article about this:
https://www.comicsbeat.com/inside-the-dc-diamond-split/
It is worth reading and I think key thing is this:
Timing is still questionable, but at the same time I have a theory on why now. Lets take couple of assumptions that are more or less supported by recent leaks and articles:
1) Higher ups are not thrilled about how little (by comparison obviously) money monthly comics bring in.
2) People running DC wanted to switch from Diamond for a while now.
Now, how DC people would sell it to higher ups? Single point of failure is bad, but creating new distribution channel, at least at first, will result in lower sales/profits and will require some investment to kick start the whole thing. Now, if higher ups aren't fond of the sales under Diamond then I can't see why they would be thrilled for this either. If they don't care about monthly comics you aren't going to sway them with arguments about how bad Diamond is and that it is stifling growth of the industry. They probably don't think that this industry can grow at all so any further investment into it probably sounds crazy to them.
So now we get a crisis. Diamond stops distributing comics and says that they'll pay only 25% of what they owe to publishers for few months. Suddenly DC people got couple of clear-cut arguments for why things have to change. Higher ups might not care that Diamond is bad for industry, but they do care that for some time they'll receive only 25% of the already small profits. Could DC have waited till the crisis ended? Possibly, but by that time higher ups might have changed their opinion again since things, likely, would have gone back to normal anyway.