It's unclear why Amazon.com ended the test. It's possible the company removed the option since Monday was also the start of its Black Friday Deals Week.
Amazon.com's statement regarding ending the test was:
We regularly test new options for our customers. At this time, we’ve completed our test and are no longer signing up new customers for Amazon Prime monthly memberships.The $7.99 price made the most sense if Amazon.com was trying to attract more folks to its Prime Instant Video service, which contains a subset of the titles available in the Amazon Instant streaming video service.
A $7.99 monthly subscription option would be comparable to other services, such as Netflix and Hulu+.
However, Amazon Prime also offers free two-day shipping on items shipped from Amazon.com or "fulfilled" by Amazon.com. It also includes access to the Kindle Owners' Lending Library, which allows U.S. Amazon Prime members to choose from thousands of books to borrow for free, as frequently as a book a month, with no due dates from a Kindle device.
That gives Amazon Prime an advantage, but the fact that Prime Instant Video is just a subset of Amazon.com's total streaming package is a negative. It isn't clear if Amazon.com will re-add the monthly option in the future.