It can have many reasons. It can cause diplomatic and PR controversy to test out MIRVs. I believe the same reason was why Pakistan didn't make it's nuclear bombs public until India tested them. Chinese military is very secretive. Why?
The reason why I say this is because time and time again, Pakistan has done tests which no one saw coming. Nukes, Babur, Shaheen II, A-100s, etc were never expected and surprised everybody.
There is a big difference of making/planning and already made.And i said the 2nd one.And u can't compare secrecy of building ist nuclear bomb anyway. When u test BM missile, thats a diplomatic news and having MRIV is a technical matter. What I want to say pakisthan may be trying to make it, but not yet made and tested.