Why is the Kindle Unlimited Fund growing at a slower and slower pace?

Kindle Unlimited is a subscription service. Readers pay $10 per month (lower in countries outside the US and Europe) to get 'unlimited' reading. They can borrow and read any book that has been included in the Kindle Unlimited program by the author or the publisher. Each month, Amazon pays out money to the Authors and Publishers enrolled in the Kindle Unlimited program. Money is paid out from a...