Why did _Research in the Mathematical Sciences_ change from open access to subscription-based? The journal Research in the Mathematical Sciences was founded in 2014 and originally published by SpringerOpen, a division of Springer supporting Open Access journals. In the first article of the introductory issue Ken Ono emphasizes the journal's commitment to Open Access publishing (albeit with a funding structure based on authors paying article-processing fees).
But if you go to the old website of the journal, you are now greeted with this message:

As of January 1, 2018, Research in the Mathematical Sciences  (RMS) has transitioned from an Open Access journal to a subscription-based journal with a hybrid Open Access option. RMS continues to be published by Springer Nature and an archive of all articles previously published in the journal is hosted here. All submissions going forward will be considered for the subscription-based journal. Please see the RMS website for more information.  

I find it very surprising that within 4 years of existence this math journal had to change its fundamental publishing philosophy.
Is there any more information, beyond the above quote, about what happened to Research in the Mathematical Sciences and why it abandoned its original Open Access vision?
EDIT: As mentioned in the comments, essentially the same thing happened with Research in Number Theory, and Ken Ono's response also addresses this journal as well.
 A: this thread was forwarded to me. Together with Springer, the RMS and RNT Editors decided to abandon open access as very few authors have federal funding that pays publication charges. The decision was not based on a low number of submissions (note. the oa goal was to publish just 25-30 papers annually). The decision was made in response to the realities of grant support in mathematics. To this end, Springer agreed to maintain the idea of low cost publication (at the Editorial Board's request), and so I am pleased that both journals have an annual subscription fee of only $99USD. Moreover, RMS and RNT are automatically included in existing bundle subscriptions. There has been an upsurge in submissions, and the editors are pleased with the papers it is receiving, and as a result we anticipate increasing the number of published articles per year to 50-60. For example, in RMS the last few years pure mathematicians who have published include Francis Brown, Bill Duke, Ben Green, Dick Gross, Michael Harris, Kannan Soundararajan, Terry Tao, Richard Taylor, Yuri Manin,...., and in applied mathematics papers by Tom Hou, Andrew Majda, P. Souganidis, Richard Tsai, E. Weinan,...
Please consider submitting strong articles to both journals. 
Best wishes,
Ken Ono
