I can see how that would be disappointing but it truly is first come first served. It sounds like someone else beat you by minutes, or seconds.
If LA allowed the items to stay in your cart for any amount of time it would just make things more frustrating. Think through how that would actually play out on the site.. fish there, fish gone, is it sold or just in someone's cart? Wait around 10 minutes, fish still not there, is it sold or is it in someone else's cart? Etc etc. The way it is now seems to be the best for everyone, and fair.