I can’t tell you why but I’d guess it’s due to supply chain issues. Or maybe they’re waiting on the arrival of this year’s model. One of the REI folks will have some insight
What I can say is I jumped on it right away last year when it was available and I’m glad I did. It’s a great tent that’s been serving me well. I think it's also one of their best selling models so I would be surprised if they dropped it.
Hopefully it will be available soon.
Thanks for reaching out!
At this time, the REI Co-op Half Dome SL 2+ tent is simply out of stock. We are hoping to see more of this item back in stock sometime near the end of June and throughout the summer. That timeline is an estimate and can be influenced by many factors. Your best bet to know when it is back in stock is to keep your eye on rei.com around that time.
Hopefully this helps, thanks!