It was a very last-minute announcement and release on Google/Apple, then the Fire TV app only becoming available on the afternoon of the first match of the season. I think take-up may improve as people realise they can follow their own club's journey through the season, rather than the previous years of only being able to watch what Sky choose to air each round.
Sky currently showing all matches live will also hold back uptake from their subscribers, as will whatever streaming service they introduce. We're actually a bit oversupplied at the moment, which is something we haven't had to get our heads round before.
Some RL-only fans who currently have Sky Sports may do the maths in the next off-season and decide to join up, once they see the feedback about how well the platform works.