If you click on this link and scroll down the page, you will see a chart of closing dates for candidates to file in each state. No one could sign up now and win the presidential primary with this many states already closed. That is why Biden cannot enter the race now and win.
http://ballotpedia.org/Important_dates_in_the_2016_presidential_race
Several things: It is important to remember that: There are 3,636 pledged delegates in the 2016 cycle. Under the party's delegate selection rules, the number of pledged delegates allocated to each of the 50 U.S. states and Washington, D.C. In addition, the current 704 unpledged superdelegates will include members of the United States House of Representatives and Senate, state and territorial governors, members of the Democratic National Committee, and other party leaders.
So, about 20% of the delegates are not chosen thru the primary process. If Biden enters late and Hillary drops out, her delegates could switch to Biden at the convention.
Now, in the event of a deadlocked convention, other candidates could emerge. The convention could hypothetically turn to a candidate who didn’t win the largest number of delegates — or who didn’t win delegates at all.
But let’s not assume wild hypotheticals: Not since 1968 has either party picked a nominee who didn’t participate in the primaries (the last one was the Democrats’ Hubert H. Humphrey). Moreover, we haven’t seen any true convention drama since 1976 for the Republicans (the tight Gerald Ford-Ronald Reagan contest that went down to the wire) and 1980 for the Democrats, when a fiery Ted Kennedy tried to pry loose enough Jimmy Carter delegates to steal the nomination.
If the Dem establishment wants to select Biden, it won't matter whether he enters the primaries or not. If they can't have Hillary, then Biden will be selected.