Very simple code-wise. Not so simple in process.
Every change we make needs to go through a build process that takes hours (like most game development) before it can be tested in a full version by QA on their machines on various devices. If I make a change on Monday it likely can't get tested until Tuesday morning. If something isn't correct, then we have to go through the cycle again. A simple change can have a minimum of 1-2 days of turnaround even if things already go well, which means getting stuff in earlier.
Additionally, we have to figure out which changes to make. What's going to be best for improving the meta with minimum disruption to the players that like the card or decks we're affecting? If a deck is the problem, should we hit multiple cards a little in that deck or one card a lot? Which one? Even in the case of Champion's Strength being a clear target, how to nerf it? Should we lower the stat gain? Increase the mana cost? Both? Change the keywords? Maybe make it significantly weaker in stats but LOWER the mana cost? Multiple this across all decks that we could consider hotfixing.
Once we pick a direction, how much time should we spend playtesting to confirm it's a good approach? Playtesting takes time after all, and often we're trying to adjust win rates by under 10% for a deck archetype. If you play just 10 games with a deck that has a 58% win rate and 10 more with a deck that has a 49% win rate, the average win rate of each is 5.8 and 4.9. That's an average difference in just 1 win, and there's actually a decent chance the 49% deck matches or beats the 58% deck in results in this kind of sample size. If the two players aren't of exactly even skill level (which they never are on a dev team) the results are further warped.
And we aren't planning on delaying other important stuff for a hot fix if at all possible, we want to find time to work it in among other things. :)