Cloning and its derivatives are mostly a forbidden and nigh forgotten science in the Imperium. Aside from the practicality remember that the recruiting and implantion process is guided by ritual as much as science. The Chapters have been doing their thing for up to 10,000 years, that's a huge amount of custom and tradition to overcome. Recruitment and implantation has become a scared process, vital not just to the manpower of the chapter but its spiritual health.
Though to be honest there's no real need for it. A Chapter's size is artificially limited by the self imposed restrictions of the Codex Astartes. There's more than enough recruits available. Look at the Astral Claws, recruiting from a 'normal' civilized world they were able to enlarge their chapter three times over within a short period of time. Think about it, planets have populations in the millions or billions. Even if 1% or even a tenth of a percent can become Space Marines that's still thousands of recruits available, more than enough for any chapters needs.