Because how does he know he can trust someone? He never got himself an assistant or apprentice because he never needed one, since he had Megaman and later Roll and Auto. He never needed to hire anyone. And by the time he built X, he was way to old to take someone under his wing. It was just too late. Just like X needed time for his reliability to be confirmed, an assistant or successor also needs time to be judged. Time Light did not have. plus since when do you hear of a scientist just giving away his secrets to the next in line? No, you don't, because they don't. The leave behind Notes, and works of their trade, but never their deep and personal secrets of the trade. They just leave behind the basic parts of their research, and the slightly complicated bits that are publicly showable. Plus, the technology and knowledge to create a robot the likes of X? Are you crazy? Why in God's name would that be a good idea? What if someone like Wily got their hands on it? Wily may be the one who set out on world conquest, but there are other people who would use that technology for dark ends. organizations who would create robots for war or sell it to the highest bidder. X was barely ready for his birth era, why would Light leave behind his inner workings as well?
Also, other scientists?
Who's this guy, Chopped Liver?
all because they were not shown, does not mean there were not any. where do you think Mets came from? and every other enemy NOT made by Wily? (any stage enemy that's just a reprogrammed worker robot)
plus,
ZERO SERIES [chameleon sting]er, HAVE YOU PLAYED IT? 
That.