Mainly because I could not open the trunk with the bag in place...I had to unstrap the bag and actually take it off the seat every time I opened the trunk...and I use my trunk often.
In addition, the bag is not waterproof...that's why they suggest using the liners.
So I went to a waterproof duffel. I purchased a small size and that turned out to be plenty large enough. But I did make a mistake by not purchasing one with a purge valve. Most waterproof duffels are the fold-over type. Because they are waterproof, they are also air-tight. Once the top is folded over, it becomes difficult to roll closed. A purge valve helps greatly in pressing out the excess air and making the duffel smaller.
Had one for about 4 years. Traveled all over the country. Never remove it except to take it in at night and I get in the trunk most every stop. Just push a little. Never used the rain cover and have ridden for many days in heavy rain. Sprayed it with a can of silicone waterproofing I got at WalMart. Makes a good backrest too. Used so much it is starting to fade from sun exposure.