The MBA and Mac Mini use different hardware. Mini updates often come months after a MBP update.
Or perhaps this will change? Maybe Apple will start using MBA hardware in the Mac Mini? I hope not, but it does make a lot of sense to me, since the Mac Mini is not a "pro" product...
Mac mini could have several options low end could be like air, high end should be like iMac. Computer was ahead of its time when it came out- so small, and performance compromises were made. Now with the miniaturization in general, fewer performance compromises need be made. I am afraid that the reason for the delay is that they are shrinking it even more. This would be so dumb. Apple pursuing "look at how small it is" for the sake of design, but at the expense of performance.