wtf is wrong with people?! You've been here since 2016. You know how this works.
Not really though; solo content never came out one month after the expansion release, but two weeks later. Here's at the very least one month away.
Now, answering the OP question:
The reason is pretty obvious; it's both about player engagement and player investment. Other comments already talked about the engagement part, which is pretty relevant; after one month and a half that a new expansion is out, the game starts bleeding out players who get bored and who don't have the collections to try thousands of options and who don't care about the grind. However, if you inject the game with more content, you manage to retain those players before they start sighing from boredom.
That's reason number one. Reason number two, player investment, is undoubtedly much more cynical; it's because if the pve is one month later than the expansion, you will be more inclined to spend more on packs right away - I mean, who the hell would want to wait for a month to spend all their gold? Psychological impact is very important; players tend to want to take part in the hottest trend, and waiting for a month so that they can earn those 12-15 packs spending 2.8k gold isn't an option. They would rather spend it all now, so that they can get in the expansion right away, and earn gold all over again in the span of time that takes for the pve content to fully come out to pay for that one. What this really means however is; players will spend the gold they will earn through the months of April and May on the PvE content - because they spent most of what they had when the exp first came out. Of course, this will translate to not being able to open as many packs as they're used to once the new expansion hits the game on August. And that will translate in an even harder time if they repeat this PvE model during September. Briefly, they will make it so that psychologically you'll want to jump on the newest thing - the expansion - making it harder to pay for the next newer thing - the pve content - that will make even harder to pay for the following expansion, making it so that you either start paying up more than usual or you just choose whether you want to go all in on PvE or PvP