Probably because Google is actively and frequently banning many Piped or Invidious hosts, and is generally currently at war with “alternative frontends” to YouTube in an effort to make users browse YouTube directly and consume ads there, or buy YouTube Premium. This is in line with their current fight for more ad revenues across their products and services. You probably have to either search for another public instance which isn’t banned (yet) from accessing YouTube, or host your own instance.
Just FYI I installed the apk from the github repo (not the google play version) via Obtainium a few days ago and it tried to make a connection to 2 cloudflare IPs during setup of my account. Without prior consent or any mention. So just be aware that there is still some form of telemetry or unwanted connections happening, even though they removed the telemetry flowing to Mozilla’s own telemetry endpoint. K-9 had zero of this, it just spoke with your mail servers and that was it. So be careful and block outgoing app connections by default. I did not analyze the data being sent, just that there were those 2 unwanted connectiins happening.