I'm sure the timing has a lot to do with it. Teams need time to ensure the new bits they put on aren't going to fall off after 20 laps.
My guess is most teams are ready to go, but just didn't have time to ship ready cars to China.
There are some major technical problems to be overcome by teams who want to convert to the true double deck diffuser. The following was taken from a article by Mike Gascoyne on the BBC website and it explains some of the difficulties faced.HOW QUICKLY CAN THE OTHER TEAMS CATCH UP?
The problem teams have in reacting quickly is that they will not only have to change their diffuser, they may also have to redesign their gearbox and rear crash structure.
A new crash structure means new crash tests, which can be quite a long process. For some teams, this could mean one or two months instead of one or two weeks.
Toyota and Williams launched their cars in January and that was the first chance for their rivals to get a good look at the diffuser design.
If they decided to react then, they will already be quite a long way down the development road.
For example, Renault may have been arguing in court about the diffuser on Tuesday but they have produced their own upgrade for use in China this weekend.
If teams were predicting the FIA would ban the design, then it could easily be up to two months before they have the new parts on their car.
Ferrari, in particular, are struggling on this front and do not expect to implement any changes until the Turkish Grand Prix on 7 June.
Red Bull will also struggle because of their unique gearbox and pull-rod suspension design. They will need a major update and designer Adrian Newey is working around the clock to make the changes.
I believe most teams will react quickly but will initially only gain two or three tenths of a second instead of the full half a second - but that extra time is still worth having.