This comment from the code explains the motivation behind this:

NOTE: The default OTLP protocol is different for declarative config than for env var / system property based config. This is intentional. OpenTelemetry changed the default protocol recommendation from grpc to http/protobuf, but the autoconfigure's env var / system property based config did not update to reflect this before stabilizing, and changing is a breaking change requiring a major version bump. Declarative config is not yet stable and therefore can switch to the current default recommendation, which aligns also aligns with the behavior of the OpenTelemetry Java Agent 2.x+.