The Tau Manta is a much, much better expenditure than a Titan. A Titan takes centuries to build. A Manta takes months. A Manta isn't as armed or armored as a Titan, but they can fucking fly, transport troops and vehicles, and you could make a hundred before the Imperium could produce a single Warlord.
Heavy Railguns are also still quite capable of taking down Titans, especially when fielded in large numbers, which they would be.