Biden and Dodd are out of the race. The sentimental favorites of foreign policy wonks and constitutional rights wonks respectively, they just can't play with the big boys this election cycle (Girl! Big Girl too! Clinton is really throwing off the casual use of gendered language in politics. A good thing.) The Democratic Caucus rules are really tough on the second tier, with no one under 15% able to gain votes. Which doesn't leave the second tier a 5% vote total to build on. This clears up the debate platform. Just the people who can win. And Dennis.
Nobody dropping out of the race on the GOP side. They all still think they can win! One of them must be right.