As a wheelchair user I have noticed an improvement in the access and treatment of myself and my mobility needs by the public and staff within theatres and arenas over the past 20 years. There are now ways I can stay in my wheelchair whilst watching a concert or a play. I can no longer transfer into an ordinary seat and I have never been able to climb steps.
I personally prefer indoor venues, they generally tend to be smaller and less crowded than outdoor events, such as music festivals, and the facilities, parking, toliets, food, stalls are more accessible within an indoor event, in my experience.
The weather is also a factor when attending outdoor venues, especially if people have mobility issues as moving around becomes more difficult in bad weather, and getting cold or too hot can be dangerous if you cannot keep yourself mobile.