Is there any in-universe reason for this ?
In the episode 'Category 55 Emergency Doomsday Crisis' Michael scolds Tahani that the manual for the neighbourhood is only to be viewed by Architects and Janets. This implies there are multiples of Janet at any given time, potentially one per neighborhood. Other facts about Janet also seem to imply this (e.g. that she loses most of her abilities when outside of her assigned neighborhood) and it is finally confirmed in the episode "Janet and Michael".
The Medium Place was specifically created for Mindy... and it does not have a Janet of its own.
After her death, Mindy's sister found her plans and used her money to start up the Mindy St. Claire Rescue Alliance which became the largest relief aid charity in the world. Both The Good Place and The Bad Place argued over whether or not Mindy should receive credit for the charity. Unable to agree where Mindy should go, they compromised and created a new neighborhood just for her: The Medium Place.
... and the Medium place is just that: medium. Nothing in it will truly satisfy anyone.
The Medium place was created to be neither comfortable nor painful for Mindy. It has the appearance of a single beige house with one small garden in the middle of a barren plain resembling that of the Great Plains in America or the Australian outback.
When Mindy was first admitted to the Medium Place, she gave The Good Place a list of the things she wanted and the Bad place made a few modifications to the list, like how they gave Mindy her favorite beer but it is always warm, or how her jukebox has every song ever but only by the Eagles (live versions only), as well as spoken word poetry by William Shatner, which Trevor describes as deeply terrible.
There is no sign that the Medium Place has its own Janet, though it is known that there are different Janets for the Good and Bad Places. Since Mindy is the only occupant of the Medium Place, it is possible that a Medium Place Janet was not created for the sole use of one person.