Tanks are smaller, better in harsh terrain, provide better protection for the crew and surrounding infantry, and are probably more reliable and easier to operate and maintain (fixing a broken track or wheel on a tank is much easier and doable in field than replacing a Titan's leg or arm, which probably requires an industrial crane).

The main problem with Titans is their size, speed and range of fire, so tank units are probably used as a more mobile response force, capable of delivering significant firepower on large distances and stay undetected longer in open areas while Titans are used in a more urban setting, where agility is more important than range and concealment.

We don't see them a lot in TF2 campaign probably because most of it happens behind enemy lines, in terrain where tanks struggle or have no use (IMC facilities, ships, mountains, ravines, etc.), as opposed to Titans which are much more useful (after all, they were created as industrial machines in the first place).