... that everything is just so interesting.
For the current book, I started off - years ago now, I'm embarrassed to say - with a draft based on admittedly sketchy research about the Jewish community in Amsterdam in the 17th century, went on to river routes of Germany, how an Auto de Fe was conducted and the mechanics of an early printing press. I used information I already knew - or thought I knew - about the Inquisition and Reformation, and the Index of Forbidden Books, and Venice and read, well, even more stuff.
My pattern seems to be that I only start writing about a setting or time about which I already vaguely know, even if my memory is not that sharp. Then I do need to research a certain amount to get the context right, but the first draft is more about character and plot than the delightful distractions of detail.
Now I'm redrafting it and adjusting my historical timelines a little, and I've decided I really can't go any further without a refresher on Descartes and Locke (it's now 20 years since my Philosophy major and what I've forgotten in that time would fill more pages than I'm actually writing), and delving into astronomical discoveries, and then I end up flicking through long-forgotten books about Gallileo or Bruno and wondering how to make a poultice for boils.
Of course there's the re-reading histories of the English Civil War, and poring over maps of ancient cities or histories of costume, and retracing my early and sketchy research tracks rather more thoroughly.
You do all that, and then a huge portion of it you cast aside, so the reader never notices it. Although they do if you get anything wrong, which is one very good reason for doing it in the first place.
I'm sure it must be a great deal easier to write vampire novels.
Umberto Eco wrote (in Postscript to The Name of The Rose) that to tell a story, "you must first of all construct a world, furnished as much as possible, down to the slightest detail".
And also it's fun.