Plus, according to this source:

http://www.seds.org/Messier/more/m001_sn.html

And I quote:

"This star became about 4 times brighter than Venus in its brightest light, or about mag -6, and was visible in daylight for 23 days.

"Some older sources had speculated that this supernova might have been as bright as the Full Moon (or mag -12). The reason for this assumption was probably the intention to fit its 23-day visibility with older model lightcurves."

And this is an object that was 6,300 light years from Earth.

And here is an online article on that very question about why we have
no definite records of the 1054 SN from Medieval Europe, showing that
if nothing else there is no easy, single answer:

http://medievalhistory.suite101.com/article.cfm/mystery_of_the_1054_supernova

Though considering how many ancient documents were lost during the
so-called Dark Ages, I don't think it is that wild an assumption that all
pertinent records on the 1054 SN disappeared one way or another.

Recall how Archimedes Palimpsest survived to our era only by the
thinnest of miracles. Just imagine (or maybe do not) what was lost in
the intervening centuries.

http://en.wikipedia.org/wiki/Archimedes_Palimpsest