The Game of Spells Has Rules
Now these rules are not necessarily written down -- and in fact are prone to changing as time goes on so they almost assuredly aren't documented anywhere. But there are certain things that you Do Not Do when playing the Game of Spells. At least not if you value your reign if you win by that move. See, crossing that invisible social line invites those that were previously enemies to ally as with such a drastic move the entire balance of power and the very rules of engagement themselves have altered.
Blowing up a coven at their weakest while you are at your strongest is one of those unwritten rules. Yes, it is a tactically sound move but while it does show your strength, it also implies that you can't do it when your weaker. The moves you make against your enemies are as much political as they are physical.
In short, this cutthroat magi-drama has political and diplomatic undertones that keep the players somewhat civil in their dealings for their own reasons.
Contrary to what might be logic, it is not offensive strikes that are done at the coven's peak. It is the prime time to set up defensive wards and spells so that the most power can be put into your protections. Spells for fertile lands and to protect the king's castle are cast when the people are strong so that they can withstand whatever the month throws at them.
Spells that attack the enemy can be done at this time of peak power, but that is power that might not be able to be put into the wards, defenses, and blessings of the land. This could leave you open to those that prefer to attack when the tides are high.
This works doubly so if it is generally more magically efficient to be defensive versus being offensive.
As has been pointed out by others, there is a limited time of high tide power to attack with before your enemies have that high tide advantage. If you are focused on attacking on the high point, then you will likely be wide open when you lose the advantage of high mana.
Both sides will taking turns hammering each other and defending with flimsy shields, or a hammering followed by a rapid attempt to mop up whatever is left of the other side once they stop offering meaningful resistance. If they do not get the key remnants before the enemy's high tide then the rout might be turned around on them.
This is of course related to ...
Distance and Mechanics
Earth is not a small planet. Even a fourth of a world away is a pretty long distance. It is also very plausible that the distances between two places is not far enough to have a noticeable difference in power between two groups as the tides ebb and flow. Now if you have intercontinental mage bombing, then that might be a thing.
For some real-world context on the matter: Paris, France and Winnipeg, Canada are roughly a fourth of a world away and along a similar latitude in the 48 to 50 degree north range.
Not just that but how do the Mana tides even work? Sure, they are influenced by the moon (and by extension the sun and other things around us), but but how much? Is it on the level of highly tangible difference between high and low like some places with extreme tides? Is it enough to notice, but nothing extreme? Or is it a barely noticeable thing?
Power versus Control
A last thought might be that magic at high tide is less controllable due to the increased volume of it. A witch trying to cast at those times might be more powerful, but are in less fine control of what they are casting. While this might not matter for dropping a giant rock on a castle (Oops ... a little to the left), it does matter if your spell attacks are curses of misfortune, or whatever you are doing might have some manner of backlash that can flambe you if you don't have enough control.
Inverse to this, low tide might be less powerful for a witch, but they have better control over their power than normal. While it might affect a large explosive spell, low tide might be the best time to slip in subtle hexes and curses upon your enemies as the control might be there to slip one past the defenses of your adversary.