Should have been IBM Tom, after legendary IBM CEO Thomas J. Watson. But perhaps they didn't want to sully his name.
It was a bit of a market play and of course has since been completely upstaged by the invention of LLMs, but it still definitely got them the attention they wanted at the time.
But IBM Tom would at least have a rationale. WTF is IBM Bob and why does it have the same name as an old Microsoft product?