His age.  Campaigns are grueling - not sure he has the energy for one.  His recent touchy-feely scandal.

Well, Gravel does run, and he's 88 and needs a cane to walk. So that's not necessarily something that would keep him out.

As for the scandal, it was very short. I don't think it will harm much on the long run.

What Cuomo said back then is that he would like to see Biden run...he left the door open on if Biden refused to run.  He said to the media....I'll get back to you...

Cuomo doesn't deserve to be on any of this list as committed/thinking/potential candidates on this day.  If Biden declines to run, then Cuomo will think about his chances - AND only commit to run by August/September in my opinion...He's a wishful contender on this day.

Fivethirtyeight has a site dedicated to tracked primary polls: https://projects.fivethirtyeight.com/2020-primaries/democratic/

I guarantee other candidates will declare after tomorrow - no matter what JB does.  Candidates typically declare in the summer months the year before the election - it's just that the Demos wanted to start this campaign far earlier than the norm.

Well, there are certainly will be declarations of running, after all we already have over 200 democratic candidates. But major candidates is another topic. I think the debates are very important in such a crowded field to stand out. So I see people seriously declaring at latest in may. Only extremely well-known candidates might get away without the debates, say Oprah Winfrey or Michelle Obama. So I see the serious declarations end in about a month. So after Biden it will slow down, although a few names are still in discussion (Stacey Abrams, Steve Bullock, Bill de Blasio).

I see at least 2-3 major declarations (active Senators or Governors) between now and the end of August.

Also, if Biden does declare tomorrow, he's only doing it for his son Beau, who died of brain cancer back in 2015, and told his father he wanted him to run for president.

Great, gives a very good overview, not just for the national polls, but also the state polls and whom endorsed whom.

I just don't understand how their pollster ratings work. And I wouldn't have expected nearly as many pollsters...

538 has an old article with explanations of it's methodology which is still linked from the pollster rating. Therefore this methodology is still true for the most part.

Short version: basically it takes old polls from that firm close to the elections and compares it to the result of the election. The calculated difference is then compared with the differences of other pollsters for the same race or (if only few firms polled that race) with similar races. A pollster who often did better than the competition gets a good grade.

Biden is definitely the last candidate anyone is going to give even the slightest shit about. 21 major candidates is too much. Some of these guys need to fuck off. I hope to God the first debates deny a bunch of them and they take the hint and quit.

Oh by the way, didn't see this posted yet:
http://nymag.com/intelligencer/2019/04/joe-biden-reportedly-delays-wednesday-campaign-announcement.html