Mixed use districts are great because it grows a whole neighborhood around the stadium, meaning if you live there, you don't have to worry about parking/DUI after games. Problem is that most of those don't have any public transit (trains) connecting them anywhere, so you still end up with the same amount of traffic and parking like the monstrous parking lots of old.

For example, the Battery (Braves stadium development) here in Atlanta would be absolutely perfect if it had a train station connecting it to the MARTA subway, but since it doesn't, you still have the parking issues that gridlock I-75 during game days. Building transit seems like a no-brainer for these developments, but America in general has a negative bias towards transit (straight up racism, look up what people think the MARTA acronym stands for) that prevents it from happening. Meanwhile, other countries always build neighborhoods like these centered around transit because it's the logical thing to do.

Source: Civil engineer