Here's why:

* Hubble's orbit: Hubble orbits Earth, and Mercury is much closer to the Sun.

* Sun's glare: The Sun's immense brightness makes it incredibly difficult for Hubble to get a clear view of Mercury. Even if Hubble could point towards Mercury, the Sun's light would overwhelm the telescope's instruments.

However, there are other telescopes that have successfully taken pictures of Mercury:

* MESSENGER spacecraft: This NASA spacecraft orbited Mercury from 2011 to 2015, providing us with incredibly detailed images of the planet's surface.

* Ground-based telescopes: While challenging, advanced ground-based telescopes can capture images of Mercury, especially during times when Mercury is furthest from the Sun.

So, while Hubble is an amazing telescope, its limitations mean that it's not the best tool for studying Mercury.