Joe Biden's status as front-runner, right now, rests almost entirely on the fact that he is far and away the top choice of African-American voters. There are many explanations for why Black voters like Biden, but I'll add one more, rather simple explanation, to the fray:
As a group, Black voters are more liberal than the average American. But they're not so much more liberal so as to explain voting upwards of 90% for Democratic candidates. You only reach those august heights when a good chunk of Black voters who might otherwise be Republicans are Democrats solely because the the Republican Party is unacceptably racist. Indeed, there are plenty of Black voters who identify as conservative -- they just don't identify as Republicans. What that means is that there is a good chunk of relatively conservative Black voters who are Democrats. A full 30% of Black Democrats identify as having "conservative" political views (compared to 8% of White Democrats).
So given that there are actually plenty of Black conservatives, a large proportion -- maybe even a majority! -- of whom nonetheless identify as Democrats, it's maybe not that surprising that one sees a cluster of support among Black voters for one of the more high-profile moderates in the race.
And, for what it's worth, this also helps explain why Bernie Sanders -- who has the second-highest level of Black support (Biden is at 48%, Sanders at 20%) -- wins among Black voters under 35. Voters under 35, across all races, represent a more liberal demographic. The corollary to "more conservative Blacks back candidates like Biden" is "more liberal Blacks back candidates like Sanders". That might sound obvious, and in many ways it is -- what obscures it is that we mentally sharply underestimate levels of conservativism (or even moderation) among Black voters because of their overwhelming partisan lean towards Democrats.