In some sources (which, I can't recall at this point), it is said that 'vat-grown' replacement limbs and organs (for lack of a better term) are available, generally to those with a lot of Imperial Clout, but it takes a long time to do so. Thus, to get a Marine back to combat effectiveness, he is generally equipped with bionics instead.
Once equipped, the Marine is the inheritor of what is often a venerated piece of wargear with a combat service record longer than his own. In addition, if another clone limb were grown for him, he would now have to have what is in effect, elective surgery, to replace his current limb/organ.
When the Space Wolves regrew/repaired their guest's badly damaged body, I think it was implied that this process took months, or even years?
Thus, most marines get a bionic replacement, to get back into active duty.
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