Why is the V1-speed callout automated in modern commercial aircraft while VR is not?
from my experience (A310, A320 family, A340), V1 and VR speeds are often very close together, sometimes they even are the same.

The automatic callout system can only output one signal at a time; during landings, one can often hear the decreasing radar altitude callouts being overlapped. Therefore, the VR callout would often supersede the - more safety relevant - V1 callout.