The reason for an open bolt is for cooling. Thompsons and other full autos are most effective when fired in 3 to 5 shot bursts. The fact the bolt remains open helps to keep the barrel cool. The Thompson has cooling fins in it's barrel so that plus the open bolt make it a more effective weapon.
When the Thompson was invented they did lot of research and testing to find the best caliber and brass length.
The .45 ACP was chosen because of (I forget the exact term coefficient of extraction). They also included a Blish Lock to delay the extraction from the chamber. There are a lot of factors that go into the design and to really understand the process you need to get s book on the subject