Fear of COVID/bleachers
Definitely not "fear" of bleachers. They just don't really have a practical place yet since Shavuos. The tragedy complicated things on technical and legal levels too. They can't use the new building now until there's proper permits. And they can't use (and they don't want to use) bleachers that aren't 100% engineer approved. A lot of red tape. Even the stage that the Rebbe sat on last night needed to be signed off by an engineer and was complicated enough. It's not worth the money to invest in a single event of two hours. Once they can use the new building, it's worth investing in strong permanent bleachers that will have proper permits and all.
Regarding R"H, they officially have a lot in Yerushalayim, but I believe at this point, it's still subject to change.