Why do bash command line arguments after 9 require curly brackets?
Specifically, your question relates to "positional parameters." Using $var instead of ${var} is shorthand in bash. In most cases it works well. Bash variables must start with a letter or underscore. It internally treats variables that start with a digit as a "positional parameter." When bash detects a positional parameter it only looks at the first digit, which is why $10 returns $1"0". By calling ${10} you are instructing bash to look at the complete variable instead of its built-in default of the first digit.
As to why it is this way? I have no idea. Legacy implementation which has been expanded upon is my guess. "Who would ever need more than....?"