Because rifled cannons are far more accurate than smoothbore?
Actually not at all; the smoothbore design being de facto much more versatile and efficient in general under almost every aspect.
Regarding the accuracy - and taking into consideration kinetic-energy rounds only, being the main projectiles that rely the most on this parameter, for obvious reasons -, whereas rifled guns achieve projectile stabilization through internal rifling, i.e. a characteristic of the gun itself, smoothbore guns achieve projectile stabilization through projectile design's characteristics: these projectiles, in fact, stabilize themselves via fins during flight, meaning no rifling is thus necessary; however, such long, thin projectiles, are too long in relation to their diameter to develop the necessary spin rate through rifling as well.
Returning now to the aforementioned point, i.e. the accuracy, rifling inevitably slows the projectile down, leading to poor terminal ballistics; whereas, due to higher muzzle velocity, smoothbore achieves longer range, flatter trajectory - subsequently leading to greater accuracy - and enhanced armor-piercing capabilities.
In order to successfully fire APFSDS rounds from rifled guns and thus achieve comparable projectile performances, installation of a bearing between the projectile and the sabot is needed. Just going smoothbore is the easiest and best option.
Also, returning to my first point, i.e. the superiority of the smoothbore design, not having to rely on an internal rifling to achieve projectile stabilization means reducing the time and expense of rifling barrels, as well as the need for replacement due to barrel wear: a smooth surface wears out a lot less than a grooved surface, with all the logistical and maintenance advantages that subsequently derive.
Finally, smoothbore guns can fire a much - and I'm using an euphemism - wider range of projectiles, including HEAT rounds and, most notably, ATGMs, which isn't exactly a trifling strategic advantage, also considering some ATGMs having solid anti-helicopter capabilities, like the LAHAT.
Rifled guns can fire HESH rounds, which the British have always been fond of; regarded by many as the rifled design's trump card as allegedly being extremely efficient against soft and armored targets alike. Well, actually not: armor is no longer as it used to be: composite materials, add-on armor kits, modular armor and reactive armor mean spalling is no longer a recurrent phenomenon nor subsequently a reliable method to put enemy AFVs out of action; during the Gulf War, a Warrior IFV suffered a friendly-fire incident and was struck by a HESH round. Most of the blast was absorbed by the add-on armour, and only the driver was injured. The only Challenger 2 destroyed in action was lost to, once again, friendly-fire and struck by a HESH round; however, the MBT was hit on an open hatch, which was shattered into deadly fragments which subsequently set fire to some HESH rounds, igniting in turn the bagged charges. I would say that was mostly due to mere bad luck than because of HESH performance.
Against soft targets a plain HE round is better, against concrete a delay-fused semi APHE, like the M908, is better under almost any aspects.
Oh, and to put the cherry on the cake, even the British acknowledged all this. It's not a case that a proposed upgraded Challenger 2 variant directly derived from a Challenger 2 fitted with the Rheinmetall L55 and still retaining all standard L30A1 gun equipment: early trials, in fact, apparently revealed that the German tungsten DM53 round was more effective than the depleted-uranium CHARM 3. However, the upgrade program was never carried out due to the lack of any current need for an improved version and because of an alleged lack of funds.
The rifled design is but a conglomerate of compromises clumsily holding up an outdated concept.