The point is that it's not your average medieval polity. Novgorod was a merchant republic in which literacy was seemingly held in high esteem in and of itself, resulting in a social pressure to acquire it. And then on top of that they had access to plenty of birch bark, which happens to be a very convenient and cheap writing material where it's available, and does not require any ink or fancy tools to use; just a sharp stylus.

As far as survival, because of how common birch bark notes were in Novgorod, they were used for mundane everyday matters like instructions to servants to purchase such and such goods, or even just a one-liner greeting, and a lot of those were then discarded and ended up buried underground. So, again, a very different slice compared to the more typical story of expensive manuscripts in libraries, or personal correspondence of nobility.