Slaves were seen as inherently unreliable. It was believed that they would willingly perjure themselves simply on a whim. Kind of a "they wouldn't know the truth unless it was forced on them." A famous Roman woman whose name escapes me at the moment (she was a major theme in Cicero's ad Milo trip but I'm on my phone so I can't look it upl) actually freed her slaves possibly to save them from this fate if they were "subpoenaed", this was the late Republic but clearly it wasn't seen as a universally accepted practice.

Source: Professor Garland's "Great Courses" lecture on "The Other Side of Ancient Life" for the first, my spotty memory for the Cicero speech, which I will try to back up when I get back to a computer