One SIM card stall owner around Ap Liu explained to me that HK-based dual-number SIMs were seen as a rather large loophole to get around the mandatory real-name registration policy on the mainland and thus were cracked down upon. Except that the SIM card stalls (not the carrier shops themselves) that hoarded them before are still allowed to sell what they have left (at a premium- I saw one place in that area selling dual-number CMHK SIMs for HK$230).

Data-only SIMs are unaffected, so those are still fairly easy to find.