As you're new to streaming, you'll find out that many series, movies, and content are carried by various services for defined periods of time. The owners of the content often do not have streaming services. The owners and distributors of Rookie Blue don't have a streaming service, and enter agreements with those services, including Roku Channel, to carry content. When those agreements expire, the content can no loner be carried. Whether or not Roku and eOne (the distributor of Rookie Blue) come to an agreement in the future is unknown. Perhaps Roku didn't feel there was enough viewership. Perhaps eOne thought they could get a better deal elsewhere. Perhaps something else altogether happened. We don't know, and it's likely we won't. There are way too many TV series in existence for each to be addresses.
It'll show up again on the Roku Channel, or another streaming service, in the future. It may be on another service already. You'll discover that it's common for movies and TV shows to appear on multiple ad-supported services at the same time, or even disappear for extended periods of time.
DBDukes Roku Community Streaming Expert Note: I am not a Roku employee.
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So, shortly after posting this in April. RB came back. A week or so ago there was a courtesy banner added that RB leaving soon. So I bombed the heck out of it but fell asleep before the last two episodes of ther final season. I figured on the 29th of the month (today) it works still be there to finish up. IT'S NOT!! IT'S GONE! SO bummed out, only 2 dang episodes to go too. Soooo frustrating. Next time say WHEN leaving soon actually is PLEASE!!