Short answer: Its a more "appropriate" cuircuit to amplify and record guitars.
Long answer: A Guitar (just like a Microphone) is a passive electric device with a very
weak signal provided by it's magnetic pickups that needs to be amplified to acceptable levels in order to be understood and recorded.
The thing is, amplifying a Mic is easy, super standard, done by the book, a problem so solved that we have been play with coloring the signal for different tastes and results for the last 30 years.
Now, Guitar amplifying, it's not only a controversial, mystical, complex, unease and very unsettling subject to opinions (to the point of being philosophycal), but also propense to a lot of snake-oil selling and fundamentalism by "Classic Tone" preachers, which demands
that the correct way of recording a Guitar is the way they did in the 60s: A very loud tube amp pushing forward low wattage, narrow speakers, with a microphone up-close and a tube pre-amp/compressor between the Mic and the console/tape. "Everything else is utter bullshit, and solid state amplifying is crap" they say.
Since this is most of the public opinion, the industry completely neglected a proper, digital, solid state amplifying method for guitars, and many of the ADCs on the market that deals with Instruments DI (Direct Input) use their Mic circuits for the guitar aswell.
In recent years, home-studio equipment become quite good and accessible to all kinds of wallets, along with VST modelings becoming competent enough to be trusted in serious production, and all this allies to the fact that Youtube and the Internet is making more and more "bedroom musicians". This results in the industry now finally attempting a proper way to amplify and record a guitar signal in a simple device with competent electronics, instead of relying on the traditional methods.
No more using a loadbox to waste into heat the 50w signal of your amp (so you don't break your windows and door away) into a 12" speakers mic'd into a tube Mic Preamp connected to a Mic interface.
EDIT: A picture (from Google) of a Guitar player home setup, for illustration purposes.