But strings are often the most common case (or even, the only case that is needed). And they need much less punctuation. Compare:

    a=1 b=2 c=3

with

   {"a"="1", "b"="2", "c"="3"}

the json version needs 19 punctuation characters just to define three variables, against the bash version that only has 3. Which one would you prefer to type with your keyboard?