Why did the chinese guard the secret of silk production so carefully?

The production generated great wealth for China as long as it was not shared.
Since China was the only country that knew how to produce silk at this point in time, they could only produce so much at a time. Silk was high in demand during this time, and if there was only one sole producer of it, naturally everyone who wanted to buy silk would go to the producer to get it; that means most of the wealth would go to the sole producer (In this case, the sole producer is China).

Sponge bob ate the crab