Cost is a big factor. Costs to buy the avionics. Costs to get the avionics fit into your probably already full instrument cluster. Costs to troubleshoot during your annuals...
Then there’s the I don’t need it because I use foreflight. Or because they regularly fly a jumbo jet that has TCAS already installed.
TCAS doesn't help with terrain it's designed to avoid collisions with other planes. GPWS or its equivalents does this.