Who decided to up the wonder quota by suggesting the lake had a remarkable coincident of 60 islands, lakes and streams is unclear. It is certainly true that, even today, there is some debate about the number of islands in Loch Lomond, though it is unlikely it reaches 60. You can read about the islands here.
In addition to the geographical wonder, however, the lake has a mythological nature as well. Eagles were often prophetic birds in the Dark Ages1, and the eagles of this lake were meant to come together when trouble hit Britain2.
In addition King Arthur is said to have starved to death Scots and Pictish warriors who sought refugue from him on the islands as he tried to clear Britain of barbarians3. It is hard to doubt that one of the reasons the lake caught the eye of people in the Dark Age was its defensive qualities. Lake edge dwellings on artificial islands known as "Crannogs4" were in favour in Scotland from 5000 years ago to almost the present day, in part for their strong defensive capabilities.