Because China is BIG. More than 4000 km from East to West, to be exact. To hit targets in Inner China with 3500-km range "Dark Eagle" missile, you need launch platform relatively close to coast.
For example, to reach new Chinese ICBM fields in Yumen, you need to be within 1000 km of China coastline. By modern standards, its rather close. To hit Urumchi, you need to launch fro, Yellow Sea. And the actual situation would likely be even worse, since "Dark Eagle" is hypersonic glider, and slow down with distance, becoming more vulnerable.
It could be argued, that submarines would be a better platform, true. But its not exactly clear, how long it would took to adapt Virginia Payload Module to CPS, and how long to sertify the missile for underwater launch. Also, there are doubts that submarine within 1000 km of China coast would be able to survive and retreat after launch - unless covered by surface warships. But there come the question: if you are forced to send surface warships near China coastline anyway, then why should you add submarine? Instead of simply firing CPS from surface ships themsevles?