Primarily size. When you have 256k or less to work with, you don't have much choice.
Specific to Erlang: http://www.erlang.org/faq/implementations.html (see 8.9)
Specific to Erlang: http://www.erlang.org/faq/implementations.html (see 8.9)
It's easy to forget how small the RAM budgets still are in some parts of the embedded world.