You're right that no one can attend but it's way more annoying than that. At negotiating rounds TPP opponents, who aren't allowed at the actual negotiations, often hold events in the city (talks, workshops, etc) and invite the delegates to them. A surprisingly large amount of them actually attend (often if there's a lunch involved as we all know how bad conference food is).
Moving it at a late stage like this disrupts all the plans of the opponents as they scramble to find new venues and new accommodation. Moreover, moving it further away from the Pacific (the second P in TPP) means opponents from countries like (for example) New Zealand, can see their airfare costs go way way up.
It's also not the first time this has happened. Last year talks were held in Japan but no details emerged of where or when until it was too late to plan anything. In Brunei as well, dates, times and locations were all changed at the last minute meaning opponents were often left (almost) literally scrambling from place to place.
Full disclosure: I'm part of the Fair Deal network (Consumer NZ) and spoke at the Brunei round of negotiations last year.