It’s a preference that may be independent of cultural norms, according to Psychologist Dr. Peter Walker of Lancaster University:

“What artistic conventions are used to convey the motion of animate and inanimate items in still images, such as drawings and photographs? One graphic convention involves depicting items leaning forward into their movement, with greater leaning conveying greater speed. Another convention, revealed in the present study, involves depicting items moving from left to right.”

and:

“It was the inspection of the availability of italic fonts in Hebrew that suggested an additional artistic convention for conveying motion, based on a fundamental bias, confirmed in the present study, for people to expect to see, or prefer to see, lateral movement (real or implied) in a left to right direction, rather than a right to left direction.”

This bias may also account for eastward travels in side-scrolling video games — keeping in mind that Mario hails from Japan, where pages are flipped from right to left.