The open world aka freeroam mode was orignally intruduced in HO if i remember right and later spread to the other versions. The problem is, that this versions were made quite close to the shutdown. The PT version we are currently playing on is a a few years older and doesnt have all cars/maps/modes/features that the games back then had.
The reason why an older version was picked, is that this version is the most stable and bugfree version that existed. While its more or less possible to import cars and maps from newer builds, porting entire modes is harder because the game version isnt compatible out of the box with newer gamemodes without adding entire new functions, adding maps and props the modes use. Depending on what gamemode is being worked on, it is possible that the source code of the mode or a part of the pt sourcecode has to be rewritten in order to support the gamemode. Ofc especially the rewritting of codes can take ages, so this will happen more later than sooner because the current plan was to addback as much content as possible.
As stated in the Roadmap, jack plans to bring somekind of freeroam back (mayby even on that map), but it wont be the same way like back then, since it was a bugfest and hated by many players.