It's not always cut on the bias. It's a personal choice.
Advantages of bias binding:
1- Great effect if the fabric is straight stripes - they'll end up diagonal. I've used this many times for a really striking look.
2- if your quilt is going to get a ton of wear and will be washed often, the binding might start to fray. If it's cut on the bias, it's stronger. Imagine if it were cut regular (straight) and wore down a bit - the entire side would be vulnerable because it would be the same string of thread (from the warp/welt) running down that strip. Does that make sense? I don't think I explained it well.
Advantage of straight bias:
Easier to cut and estimate fabric needed.
I think it's good to understand both methods and how to work the features so you get the best result for your particular project (and fabric).
Some say it's easier to work with the bias binding. I didn't really find any differences between the two. Also, I've seen people got their binding along the length of the fabric, not width, so they don't have to join the strips.
Again, it's a choice. Personally, I'd rather join the strips than deal with cutting 108" of 2" strips. Unless maybe there was something very special line design/pattern on the fabric. Maybe.
I'm sure others will have more to add to the list.