Why does Mindhunter use fictional main characters?

From an interview that Time magazine had with David Fincher (executive producer), when asked how closely the TV show adheres to the true story:

Joe Penhall (writer and producer) was the first person to say, “I think I can do a better job of dramatizing this if I’m given the leeway to take some of the attributes of this person and some of the attributes of this person and create a new character.” So that’s what we did.

So, just as several comments have suggested, the TV series is only based on the book Mindhunter, specifically so that the writers and producers would have an easier time in creating the series, and not have to conduct extraneous research or risk inaccurately portraying certain events.

In an attempt to restore any disappointment that may be experienced by this fact, Fincher then went on to say,

A lot of [the interviews with serial killers] were taken verbatim. The Kemper interviews, the Manson interviews — that stuff is pretty well documented. We stayed as close to that as we could while still having a dramatic arc.

That being said, not all of the show is interpretation, but instead, is a kind of recapitulation, so to speak.