Starting with Daredevil, because it's the first of the Netflix Marvel shows to come out...
In April 2013, Marvel Studios president Kevin Feige confirmed that the
film rights to Daredevil and his associated characters reverted to
Marvel from 20th Century Fox in October 2012, allowing those
characters to be used within the Marvel Cinematic Universe. As
explained by head of Marvel Television Jeph Loeb in 2015, Marvel
Studios had "first dibs" on the character once the rights had
reverted. Drew Goddard pitched a new Daredevil film to Marvel, but
Marvel was not looking to create an R-rated film, and Goddard did not
want a "watered down version" of the character, as he also explained
in 2015: "I went into Marvel and talked to them about making it as a
movie a couple of years ago, long after the Affleck movie. But what we
all sort of realized is that, this movie doesn't want to cost $200
million. The thing about Matt Murdock is, he's not saving the world.
He's just keeping his corner clean. So it would feel wrong to have
spaceships crashing in the middle of the city. But because of that,
Marvel on the movie side is not in the business of making $25 million
movies. They're going big, as they should." Marvel Studios
eventually decided that the character would be better served in a
So essentially this push from Drew Goddard to keep Daredevil "truer" to the comic adaptation, allowed for Marvel Studios, a subsidiary of Disney since 2009, to save this project for a television series, instead of making it a film in part due to the kind of budget Goddard wanted.
In October 2013, Deadline Hollywood reported that Marvel was preparing
four drama series and a miniseries, totaling 60 episodes, to present
to video on demand services and cable providers, with Netflix, Amazon
and WGN America expressing interest. A few weeks later, Marvel and
Disney announced that Marvel Television and ABC Studios would provide
Netflix with live action series centered around Daredevil, Jessica
Jones, Iron Fist, and Luke Cage, leading up to a miniseries based on
the Defenders. This format was chosen due to the success of
Marvel's The Avengers, for which the characters of Iron Man, The Hulk,
Captain America, and Thor were all introduced separately before being
teamed up in that film. In December, Goddard was officially hired
as executive producer and showrunner for Daredevil, but Marvel
announced in May 2014 that he had stepped down from the role to focus
on directing a feature film based on Marvel's Sinister Six for Sony
Pictures Entertainment. Goddard, who wrote the first two episodes of
the series, remained with the show as a consultant and executive
producer, while Steven S. DeKnight took over as showrunner. Marvel
revealed that the series would officially be titled Marvel's
Daredevil, with DeKnight, Goddard, Loeb, Jim Chory, Dan Buckley,
Joe Quesada, Stan Lee, Alan Fine, Cindy Holland, Kris Henigman, Allie
Goss, and Peter Friedlander serving as executive producers.
So at some point we have to presume the EPs got together and said let's not just do one of these darker neo crime noir takes on just one character, let's still follow the format of The Avengers and create multiple series and crossovers leading to The Defenders.
To try and say why "exactly" they went in this direction seems to be unknown, but creatively I think they just saw potential to branch out the franchise by offering a different tone/aesthetic/taste and medium (streaming service), which seems to be mostly successful.
Fox began to do something similar with X-Men by adding some nostalgia and appealing to a younger audience with the revitalization of First Class and Days of the Future Past, but also with spin-offs like Legion, Logan, Deadpool, and New Mutants. It's just a way to bring other kinds of viewers into the franchise, while also being more creative and redefining the super hero genre.