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.