First, there are still other choices than between Sanders and Biden. In fact, if you don't like Sanders, I recommend that you vote for a candidate other
than Biden. He's a good man but it's getting more and more unlikely that he'll win enough primaries and he'll block the emergence of alternative candidates who might be able to win.
As to Booker, his campaign didn't have the funds to pursue the kind of aggressive marketing campaign that's (unfortunately) required to secure a major party's nomination these days. He had a positive message but wasn't able to distinguish himself from the twenty other candidates running. This is what happens when the voters are offered too many choices
. Lots of other good candidates were forced to suspend their campaigns for similar reasons. Forced to choose between four or five candidates, people get to study the positions, the personalities, and political acumen of the candidates. Forced to choose between twenty, only the most well-funded and well-known candidates have much of a chance. Assuming one of the current front-runners gets the nod, it's possible that Booker might be included on the ticket; he'd make a good vice-president.