@Moth
Sorry for wall of text, guess I was bored and in a rambling mood when I saw your comment lol. And you may know all of these, less of an 'answer' to you than a general explanation for anyone curious tbh. And fk correcting grammar etc. now, tired and bored lol.
One possible reason--If they wait, the popularity may have waned some by the time the manga progresses to original story.
Also like the 'original' manga it works to both provide another source of enjoyment based on the original story, and it can allow readers of said novel to enjoy farther along portions, as well as working as a kind of advertisement for the novel to those who may not usually start a novel; like those preferring manga, but get hooked on this and/or it's parent story, enough to make them interested in the original novel.
These types of side stories can also provide opportunities to do things somewhat differently without 'wrecking' the original. These new takes, aside from possibly providing simple enjoyment for the fans and continuing expansion of the universe in which they are set- something increasingly considered favorable for an IP's success-- such as that seen in the 'recent' Marvel movies and their continuous universe; They can also allow authors/artists/etc. to 'spread their wings'/experiment more than they may be allowed to do in the parent story. Occasionally allows for entirely different styles, but with the 'stability' of using an already known property. Like the many 4koma spin-offs being produced, or Shoujo 'versions'/variations occasionally seen.
Other than those possible reasons there is the one generally most likely--Money.
Somewhat tied into reason one (current popularity) of taking advantage of its success so as to not wait too long, and having it wane-- doing so at a time like this can often increase the readership, and the profit, of the spin-off/alt/whatever, and possibly the other way around to a point.