It seems the technical issues are the problem.
You dont "host" a game like in a mulit-player enabled FPS such as FarCry2, with GT5 you open a slot in their hardware backend to "host" the game using map data that is on their hardware and our PS3's. The your online "lobby game" manages the multiple racers from that management point.
They would need to allow us to upload our maps to their backend, and that may be the issue - instead ot he server hosts having to only have 75 tracks worth of data to reference for online hosted racers, they woul dneed to somehow store and then access and manipiluate potentially 1,000,000's of tracks worth of data.
This is how I understand the hosted lobby features work. I could nad have been known int he past to be wrong though
They would only need to download the track data file at the start of the race. That would only add a few seconds at race start.
IT wouldn't take much data on the servers, all the serves would need to be loaded with are the base environments and the assets used in the scenery, the actual track is just a collection of numbers and is generated from a seed. The server would only need to upload those numbers from the hosts PS3 and then use that seed to create the same track it'self. The actual memory these tracks take up is tiny because they're just numbers.
- Translated Famistu magazine interview, http://www.andriasang.com/e/blog/2010/12/15/gt5_plans/He said that they'd like to make it so that you'll be able to use the courses you made in the game's course maker mode in online races. This is something that he hopes to implement soon.