Is Diretide late?

Mostly the reason they're mad is because of alack of content and communication from Valve in the past few months. Diretide isn't the best thing ever but its a welcome, fun mode that changes up Dota a bit.

A new map is a BAD idea. The heroes are balanced around the map. A new map would require massive changes to almost every hero, and the game would become much more unstable and a pain in general. Look at League of Legends. It's not nearly as balanced as Dota, with 3 or so maps. Now if they were to create an entirely new set of heroes for a new map, it might work. But IceFrog and Valve are not going to go through that much work for the sole purpose of providing an extra map.

Diretide is a mini-game to celebrate the holiday, its quite fun and allows users to get some easy common cosmetics. Whereas a new map that focused on changing the entire laning phase would be impossible to balance with the current hero pool, it would get little to no support and would be just another game mode that sits there and doesn't see competitive play.

And collecting candy in a bucket in November sounds like a better idea?

Yes, they already have everything for it and it wouldn't put a strain on the dev team like pushing out the constant content they have been the past 3 Internationals. How does developing a brand new game mode that would be permanent and hinder balancing for the main game mode sound better to you?

No offense but if you want a 2-lane mode, play League. Dota's entire balance system is based on the map. It'd be EXTREMELY difficult to have heroes and items balanced on two different maps all the time.