Well at the initial stage recruitment at ISRO centres (was not centralised and thus due to more awareness, local people got in.

Now ISRO has centralized recruitment through ICRB which conducts the written test all over India, and thus people from all over India are now getting into ISRO.

Though there is no regional bias in recruitment there is a greater chance that a person would be posted in one of the centers in South India because ISRO has most of its facilities located down south.

The workforce of organizations being dominated by people from a particular region owes to the fact that any large scale government facility in any part of the country directly involves and positively impacts the local/native populations.

ISRO Centers

Indian Space Research Organisation

Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram.

Liquid Propulsion Systems Centre (LPSC), Thiruvananthapuram.

Satish Dhawan Space Centre (SDSC-SHAR), Sriharikota.

ISRO Propulsion Complex (IPRC), Mahendragiri.

ISRO Satellite Centre (ISAC), Bangalore.

Space Applications Centre (SAC), Ahmedabad.

National Remote Sensing Centre (NRSC), Hyderabad.

ISRO Inertial Systems Unit (IISU), Thiruvananthapuram.

Development and Educational Communication Unit (DECU), Ahmedabad.

Master Control Facility (MCF), Hassan, Karnataka.

ISRO Telemetry, Tracking and Command Network (ISTRAC), Bangalore.

Laboratory for Electro-Optics Systems (LEOS), Bangalore.

Indian Institute of Remote Sensing (IIRS), Dehradun.

Antrix Corporation - The marketing arm of ISRO, Bangalore.

Physical Research Laboratory (PRL), Ahmedabad.

National Atmospheric Research Laboratory (NARL), Gadanki, Andhra pradesh.

North-Eastern Space Applications Centre[20] (NE-SAC), Umiam.

Semi-Conductor Laboratory (SCL), Mohali.