Send isn't a command; it's a keyword, like if, do, etc., and the designers
of HyperTalk didn't allow keywords to be trapped by handlers in the same
way that commands can be trapped.
Perhaps the simplest way to explain the difference between keywords and
commands is that keywords force the compiler to do something out of the
ordinary. In the case of "send", the target for the message is potentially
different from the default target, so the compiler has to do some extra
work to figure out where to send the message.
This article is adapted from the Claris Tech Info database.