For heka, one of the things we wanted besides performance, was a small, easy-to-distribute binary you could drop on a system and 'just run'. Go is fantastic for distribution thanks to the small staticly linked binaries you can get out of it. To accommodate the pluggable filters and such later, a Lua sandbox system was added... and oddly some of the other Go pieces ended up being faster in Lua which is why Lua decoders/inputs are now an option for heka as well.

Note: I gave this talk.