1) The Carthage was already significantly damaged.
2) A Karuna is pretty big--not a destroyer, sure, but larger than any GTVA corvette. Couple that with the absurd number of Apocalypse antimatter torpedoes, flak munitions, and the reactor itself, and you get one hell of a punch (and bang).
3) The Karuna would be ramming at speeds around 60 m/s....that's considerable.
4) The Colossus skewered that Orion with heavy beams before it even got close. Not exactly much of a ramming attack (with poor speed, to boot).
5) Square-cubed law (IIRC) implies that the Colossus would be made out of (or built to be) far, far sturdier stuff than your average Orion. The attempted ramming was at the frontal armor of the ship, which is probably one of its thicker points.
6) I'm not sure you could even call it a ramming--I always got the impression that Koth's ship was largely gutted by beams before it really hit the Colossus (as in, it blew up before any real ramming occurred).
7) If at first you don't succeed...you've got a lot more options at your disposal. Another Karuna, and/or another Sanctus. And/or the rest of the task force just unloading into the crippled Carthage with Apocalypse spam, point-blank mass-driver shells, gauss cannon fire, and even some PDT and Burst Flak fire for the cherry on top.
1. Probably not relevant for the purposes of this discussion. I don't see huge chunks of it floating off so it's retaining most of its mass.
2. The front end of a Karuna (a lot of the front in fact) is mainly its two mass drivers. In overall size and overall mass it's at best the same. This also means that the part of the ship which will make contact is much more likely to crumple and absorb energy from the impact rather than remain rigid enough to transfer it.
Also assumes a complete lack of safety systems for the munitions, which is very much inconsistent with the need to handle ordnance safely. BP has consistently gone out of its way not to paint anyone as dumb. This ammunition is almost certainly all boresafe.
3. 60m/s still isn't that much.
4. Again, it's not like huge portions of the ship necessarily disappear.
5. Square-cube law applies only in consistent gravity. No, artificial gravity doesn't count, the mere fact they have it means they can futz with the specific gravity of any portion of the ship as they like. (Hell, there may not be reason to actually have gravity in large portions of the ship in combat, or even normally.)
6. While this is possible, the reality is it usually does make ramming contact and the mission is designed for it.
7. Ramming is by definition an option of last resort unless your ship is specifically designed for it. The Wargods are a major portion of the UEF's defense. They represent an asset years, possibly more, in the making, and are not to be sacrificed casually. If it is remotely possible to save the ships and crew then
you should; only when it became clear this was no longer in the cards was ramming considered.