Well, rifled gun is just outdated technology.
Rifled gun have lower service life compared to smoothbore.
Rifled gun needs higher pressure to fire projectile at the same velocity as smoothbore gun.
Rifled gun is problematic when firing HEAT, because HEAT do not like spin effect, so to overcome this problem, designers designed a non rotating warhead mounted to the outershell by ball bearings so the shell can spin, but warhead won't. This makes projectile design more complex and also probably expensive.
Rifled guns are used because of HESH, but HESH is not a good and modern solution. First problem is it's anti-armor propersties, HESH can be stopped by a simple metal screen, because of it's working mechanism. Also HESH is not really good against infantry, because to be HESH it needs very thin shell, which means that there are less fragment of less weight that can harm infantry than in case of traditional HE round.
Rifled guns are currently minority, especially in 120mm calliber, not much modern ammunition is designed for them, actually non modern ammunition is made for them, maybe besides British round for L30 gun, but that ammo is also not manufactured any more and is a 3 piece ammo, that won't fit to the guns other than L11 and L30 series.
Rifled guns are more accurate, this is one of basic arguments of fans of rifled guns, but in fact, there is no proof that at typical distance where conventional ammunition is fired, smoothbore is accurate. In fact in all NATO competitions and tests, rifled guns were not better than modern smoothbores at such typical distance up to 4,000m where conventional ammunition is used. Beyond that distance guided munitions must be used.
Besides that, all the stories about long range fire engagements with use of rifled guns are not accurate descriptions of what really happend. For example that tanks firing at such distance were commanded by high ranking officers, normal crews can't waste ammunition in such way. ALso nobody says how many rounds were fired before one of them hit target eliminaing it.
Instead smoothbore guns offer:
Longer service life.
Easier production and costs reduction due to less of them need to be changed thanks to longer service life.
Can fire projectiles at higher velocities with less pressure.
Simpler and cheaper ammunition.
Are manufactured en masse, big variety of different types of ammunition avaiable on market.
Accuracy not worser than rifled guns on typical engagement ranges with use of conventional ammunition.
More modern ammunition, for example instead of HESH, far more effective modern HE ammo with programmable fuze, that enables for example air burst mode to defeat target behind cover (e.g. sand berm) or delay fuze mode that enables to defeat hard targets from bunkers, other structures to lighter armored targets and even older tanks (see American AMP round test photo where AMP HE round with programmable fuze defeated side turret armor of T-55 tank, this is ~100+mm steel cast armor, impressive for a HE round), such ammo is invurnable for light steel screens effective against HESH.
And many other points can be pointed out in favor of smoothbore guns.
@Kaustav.
XM1111 MRM-CE can't be really compaed to other GLATGM's like LAHAT, CLGM or others, because it is a completely new quality for such guided ammunition for tanks. XM1111 besides that can be guided by platform that fired it or other platform that see the target, can also be fired in the direction of detected enemy forces in autonomous mode, and it will find it's targets by itself.
ld:} 
