Why in the last few years in particular - that's the real question. Pre-2002, the IMCR issue rate was relatively stable.
Look at the average permit or microlight 5 years ago. The microlights were flying tents and the permit stuff looked like it had been cobbled together in someone's garage.
Now look at them, CT2K, Sky Ranger Ikarus etc on the microlight front sipping Mogas with 152 speeds makes for attractive flying. Permit wise look at the stuff like the Pioneer, the plastic thingy that Rod1 flies, the Glasair, the RV series, these are serious fast tools that people fit kick ass glass cockpits to. the Glassair even comes in a Turbine version!!
Given the option of a 172 on an expensive CofA or a 200kt hotship with a state of the art glass cockpit and able to self maintain I can see why people choose to give up rarely used IMCR privileges for far greater benefit.
If I could fly IFR Airways in one I would give up my Cessna in a heartbeat.
You also have to look at the ever aging rental fleet with ever increasing placards for inop items. Most rental fleets have only one or two IMC capable aircraft and schools are reticent to sign people out for rental in real IMC due to the risk of the aircraft being stranded away etc that it becomes harder for people to actually use the IMCR. So for the average non owning IMC pilot once the novelty wears off they realise in a rental scenario that the IMC does not have a great deal off worth. The sales of IMC capable tourers to private individuals have fallen off as the price of Avgas rises.
It's not difficult to see the reasons.