It's the phrase "homelab", I suppose. Why not call it "Home IT" or "Home Data Center" or "Home Network Infrastructure". Calling it a home laboratory sounds like enthusiasts are trying to escape some preconceived notions about the topic. I digress...
As far as the name, I think it comes from the term "networking lab" that netops actually use to trial out network topologies before deploying them in the field. It's a home networking lab, that's all.
:)