Advocate Dumisa Ntsebeza said the 20 families only recently became aware the hearings were being conducted and could not afford to travel to attend them, the South African Government News Agency reported.
The government was reportedly in the process of helping family members who wish to attend the hearings.
A commission appointed by President Jacob Zuma began the hearings Monday in Rustenburg to investigate the deaths of 45 people, including the 34 workers, at the Marikana mine in August.
The commission, led by retired judge Ian Farlam, began the inquiry by reading all the names of those who died in the violence. A moment of silence followed.
Officials from the Lonmin mining company, the police, and mineworkers unions were to be interviewed by the panel, which has four months to complete its work.
Most of victims' families live in the Eastern Cape and the neighboring country of Lesotho and "they want to believe that if they can be part of this process it can assist them in the healing process," Ntsebeza said.