I've asked about this too but it's never going to happen, and I can explain why. First of all, MIUI is designed with HDPI devices in mind. The graphics, the themes, everything is high resolution. To scale it all down to fit on the wildfires QVGA screen would take a lot of work and nobody is really willing to do it...because it's pointless. Why pointless? The Wildfire is what I would call an entry level smartphone. In order for HTC to make such a phone, they had to cut back on costs so people could afford it. In other words, the hardware is shite. The processor struggles in CM7, even when overclocked, the GPU might as well be non existent, and the screen resolution is poor. And then there's the low storage space and little bit of RAM. It would never work, unfortunately.
I was disappointed too when I heard about this, so I went out and bought a new phone.