Although it is not noticeable on a day to day or year to year timeline, the quantity and quality of magica available to humanity has slowly been decreasing. However, that is not to say there is LESS magica in existence, only that it is slowly but inevitably being amassed and consolidated by powerful beings. Primarily, demons. Demons are capable of storing shockingly large amounts of magica in their bodies, and until they use it or perish it is taken out of circulation. Not only do the Demon Masters store vast amounts of magica, they also use their organic structures and buildings to hoard even greater quantities of it. Bit by bit the demonic planes are siphoning away the magica of the human world. I am not even sure this is intentional, or merely the byproduct of beings that care only for themselves.
I can remedy this. In time.
-Further studies on Magica and Demonic interactions by the Highest of Angels