Because these forums are a hot mess. They aren’t even blizzard forums. They are copy pasted from another company and they tried to force their own rules into it. So we get lots of oddities and inconsistencies.
You need a higher trust level to include links in your post.
This would be a more reasonable requirement to include if there were any realistic way to view your own trust level, or to understand progress with it, or any other such thing. Instead it’s a black box that lets some people sometimes post links and others not.
edit: For example, the only way for me to know that apparently I’m allowed to embed youtube links now was to try it and see what happened.
Something else to note is that there are two different Trust Levels related to linking. Trust Level 1 can “Create links and video embeds to trusted websites like Blizzard websites, YouTube, or Twitch”. (Why someone thinks YouTube is trusted is beyond me, but so be it.) Trust Level 3 is necessary to “Post animated gifs, images, compatible video embeds, and links to any website”.
It’s pretty dang easy to get to Trust Level 1 with just normal forum activity: Enter 5 topics, Read (scroll past slow enough to credit as reading) 30 posts taking at least 10 minutes. I probably have more than a dozen of my alts to Trust Level 1 just because I was testing stuff and posting across them and giving loves from them.
Trust Level 3 is a lot harder to accomplish, particularly because it says we “must have visited at least 50% of last 100 days”, and 100 days haven’t passed on the new forum yet. The only way anyone has it now is if some stuff carried over from the old forums or it was given. (MVPs, I think, got a boosted Trust Level.)