It wasn't just the "series of tubes" phrase itself; it was the whole nonsensical story that followed that. Wikipedia has the full quote and a recording, but to summarize: Stevens opposed network neutrality on the grounds that it could lead to network congestion, which he illustrated with an example of an email (in his words, "an Internet") sent to him which took some time to arrive. As an argument, it was barely coherent, and demonstrated a lack of understanding of the technology he was trying to make an argument about.