What are the differences and why did SpaceX prefer to use PICA?
PICA is less dense than other materials and enables a lighter heatshield.
Previous materials like fully-dense Carbon Phenolic (heritage entry grade Carbon Phenolic) used on more extreme missions like Pioneer Venus and the Galileo probe are propbably overkill for a LEO entry and are no longer capable of being produced:
- Thermal Protection System Options For Outer Planet Probe Missions, OPAG, January , 10-11, 2013, Atlanta, GA, Presented by: Don. Ellerby
It doesn't hurt that NASA strongly supported SpaceX in making its cargo Dragon heatshield: