By delayed I mean that the committee was discussing about discriminated unions since a long time ago and it was never "the right time". You can see the discussions related to implementing discriminated unions on Github.
They also often need a lot of scaffolding to be built along the way. We like breaking hard problems into much smaller, composable, units that we can build into the language and then compose to a final full solution. We've been doing that for many years, with unions being a major goal we've been leading to. At this point, we think we have the right pieces in place to naturally add this in a way that feels right to the C# ecosystem.
Thanks for the feedback!