why the shopping cart always on right?

It's probably just to do with trend. Most likely some popular shopping website did it and new websites just felt like that's what we are now naturally used to.

More like social convention. But as we tend to ~try~ explaining things our inconscient mind receive and react to, professionals of many fields always come up with something like "its easier to see" (indeed), "important stuff should be on the right side" (it may be), "we got a natural perception that 'our right side' gives out more space than the left side" (we got a neuroscience explanation to many things concerning perception so yep, it may "give out more space"). The truth is, as everyone's wrong because there's no logic reason, everyone's right. We are now used to it, our brains can't "unsee" the logic we have assigned to the signs we create since the very beginning of times, like more than 20k years before inventing language. So when in doubt: be the fish that swims with the river, not against its flow if theres no possible reason for that. Stick with ~trends~. More like making mistakes and learning from them.