Why is the calcium in bok choy more bioavailable than in some other greens?

Because it is lower in oxalates compared to some other vegetables.

Bioavailability refers to the proportion of a nutrient that the body can actually absorb and utilize. While bok choy does contain calcium, which is vital for skeletal health, its advantage over other leafy greens stems from its lower concentration of oxalates. Oxalates are naturally occurring compounds in many plants that bind to minerals like calcium, significantly inhibiting the body's ability to absorb them. By having lower oxalate content, bok choy allows the ingested calcium to be absorbed into the bloodstream and utilized by the body more effectively than calcium found in higher-oxalate vegetables.