From the FAQ on MRAN:

Why can’t I update to or install the latest CRAN packages?

By default, Microsoft R Open offers its users predictability using a static CRAN snapshot date. For example, the CRAN repository for Microsoft R Open 3.4.4 is configured to point to a snapshot date of Jan 1, 2018. Consequently, with Microsoft R Open 3.4.4 you’ll always get packages as they were at midnight UTC on Jan 1, 2018 by default whenever you use install.packages.

Using a fixed CRAN repository snapshot means that every user of Microsoft R Open has access to the same set of CRAN package versions. This makes sharing R code that relies on R packages easier, and reduces the chance of incompatible R packages being installed on the same system. Learn more about fixed CRAN repository snapshots.

It is possible to change the default repository date.

Well that’s annoying. Running options(repos = c(CRAN = "https://cran.revolutionanalytics.com")) seems to fix it but might need to update this with every MRAN update.

See: https://mran.microsoft.com/faq#change-repos