Why were written sentences longer in the past?

Question

These ELU answers affirm, but do not explain, the decrease in written sentence length. So why?

To allow for comparison with modern dialects, I restrict this question to:

1. writing in European languages (for those who desire specificity: Germanic and Italic),

2. that have remained extensive and prevalent since the 1500s.
   _{(I picked this century, because Modern English started in roughly 1550, and Modern French officialised in the 1530s)},

_{PS: Since posting this question, I have read some explications, but nonspecialist: 4, 5, and 6.}