Well, yeah, but for the obvious reason - tweets are meant to be short and consumed in large numbers, but aren't generally meaty enough for a discussion, while some people are using Google+ as essentially a blogging platform. 140 (or 256) characters is rarely enough for an entire HN discussion, but that doesn't mean I don't enjoy reading dozens of tweets at a time to get a sense of what's going on.

Actually, many of those tweets have links, so in some sense Twitter competes with HN as a link aggregator, not with Google+ as a primary source.