There are a couple reasons.
Makers have to go through their old stock of steels. A lot of makers have a decent amount of steel that they buy months or years in advance and they need to burn through those first.
Secondly, Magnacut isn't really super understood by the masses yet. Cedric and Ada did a cut test, Dutch Butchcraft Knives did some videos on it, a few other channels like Outpost76 did some work with it. I think Big Brown Bear did some video with it, and I will link his because he is awesome.
It's a very new steel, and so once more people get a handle on it and test it, they will say what they feel about it. Also another thing is that makers do not know what to heat treat this to. Above 62, and the steel is not as durable. I suspect fixed blades will be below 62 HRC, unless they are smaller. CRK thinks 63-64 is the best hardness to have it at for folders. Time will tell if this is really the case. The data is out there, but real world use will probably determine the HRC makers will settle at.
I mean think about this as well. Like with anything, some will do it better than others, and there will be lemons with good makers. Until magnacut has been around for awhile, I don't think we will see it very widespread. I think it will basically replace most blade steels out there though in the long run. There is a reason why makers say magnacut is one of the most important knife moments in recent history. It is also easy to machine and grind according to CRK. It's as tough as 4v at certain heat treats, it's virtually rust proof, it's easy to sharpen, and it has better edge retention than s30v.