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I think it's mostly dependent on the fact that it's the most convenient way in combination with the standard jewelcase - that is, if you're right-handed. Being right-handed, it's easier to hold the case in your right hand and open it with your left than the other way around. Knowing this, it'd be odd to place the booklet in entirely differently, which would be the case if you were going to go right-left.
And as has been pointed out, Shiina Ringo does have some right-to-left booklets. Her Karuki Zamen Kuri No Hana albums is probably the prime example of this.
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