Why is Pawn "childish"?
Pawn is a small, light-weight and C-like robust scripting language that runs on a virtual machine. It has even been ported to a device of only 32 kilobytes of RAM. It is created just to be fast enough and not use too much resources (hence why many games use it).
Why doesn't HL support a SM-like scripting platform (language)?
It doesn't need to. It is an old game created in 1998 (although started in 1997) not highly-populated anymore. Since AMX
X's development in 2003, there have been a lot of modules and plugins created around it. Not even active people would bother switching and porting stuff for a new syntax.