OpenTelemetry is just an API for instrumenting your software. For actually collecting and viewing your traces, you'll need an actual implementation -- Jaeger and Zipkin are some open source options.

If you develop an open source project and one of your goals is "I sure wish my users could use Datadog instead of being forced to use an open source thing!" then OpenTelemetry is a project you should keep an eye on. The goal is to make the instrumentation agnostic to the underlying provider.

They do, it wasn't called OpenTelemetry at the time, it was called OpenTracing:

> By 2017, open source projects like Open-Tracing and Open-Zipkin were mature enough for use in polyglot runtime environments at Netflix.

I don't think OpenCensus even existed in 2017, looks to have been announced in 2018:

https://opensource.googleblog.com/2018/01/opencensus.html

Too new or not mature enough at the time: “When we started building Edgar four years ago, there were very few open-source distributed tracing systems that satisfied our needs.”

Some of their engineers have made PRs to the OpenTelemetry JS repository, but it wouldn't have been mature enough (or even existed) when much of the work described in the blog was underway.

Because they don't use it or because they "reinvented the wheel"? Perhaps it wasn't mature enough when they started this journey.