Actually, compared to the first balance patch in each of the last two years, this patch is considerably later.
Last year, the patch that nerfed Edwin and Boggspine Knuckles came on Jan. 8:
And in 2020, the first balance patch hitting seven cards came on Jan. 9:
So, they definitely can and do put out balance patches earlier than they are this time.
For this patch, the most likely explanation is that one or more of the balance changes needed to come in a client patch. Since they can only release a client patch every two or three weeks at the fastest, they needed to roll the balance changes into the upcoming big patch for BGs, Duels and Mercs. The alternative would have been to push balance changes early, but push the upcoming big update (and likely the mini-set after that) back several weeks.
So, likely, they chose the current route not because it’s a great option, but because it was the lesser evil.