There's a number of reasons.

Firstly, it's really really damn hard to hit a target on a planet from space. The physics alone is insane. Calculating your relative velocities, how fast and in which direction you, the target and the planet are moving, how long it'll take the orbital strike to get there, what affects the atmosphere will have on it, and so on.

In order to have an accurate orbital bombardment with any hope of hitting, the firing ship needs to be in geostationary orbit at low anchor. It then needs to line up the shot, fire, observe, adjust and repeat.

Doing so makes you incredibly vulnerable to counter-fire from other ships, ground-to-air weapons, and so on. It would also require you to reposition your ship, which takes time, fuel and probably needs fighter escorts, scouts and support.

It can be done, but it's a huge waste of resources so it has to be worth it.

And that's even assuming they could hit the titan. A tiny target on the ground (even a Titan is tiny compared to orbital dynamics) might take tends, hundreds or even thousands of shots.

Is that worth taking up the combined attentions of an orbital battle group, and making them vulnerable to counter attack? Probably not.

It's likely it could be better used to level a city, or keep another fleet away, or whatever else. Let something proportionate deal with the titan, like a couple of shadowswords or your own titans.

It's like saying well the enemy have a Leman Russ, shall we request a titan to defeat it? Well really it's not worth it, there's higher profile targets, we can task an appropriate response and better use that titan elsewhere.