Twitter "conversations" tend to be one person going on a rant for 10+ tweets and then attempting to respond to people who comment on different parts of said rant. The formatting on that site is terrible.
The reason for the 120-character limit on Twitter is to show who the pompous windbags are on Twitter. People who post more than 120 characters are filibustering windbags. They prattle on and on and on, they're redundant, they repeat themselves!