Why is SWAN dimming? The SWAN C/2020 F8 comet is passing near the Earth, but from Wikipedia

The comet has dimmed a small amount since May 3.
It was expected to possibly reach 3rd magnitude in May, but is now expected to hover closer to magnitude 5.


What is the cause of this dimming?
SWAN seems to be on a parabolic orbit. This means that it is the first time that it is interacting with the Sun, making it a "new" comet. New comets are covered in a layer of very volatile elements that will vaporize pretty fast when the comet is still far away from the Sun. This causes a surge in brightness. But once those very volatile elements are gone, it stops getting brighter and the comet's brightness can even decrease. 

In addition to this, it is mostly losing gas, so its tail isn't as visible as if it were a tail of dust.

(From this article, in French)