Unlike the BGA2 processors the Micro-PGA2 CPUs are not available in low voltage version.
When you compare the mobile Pentium III with the mobile Celeron at 100 MHz the TDPmax is quite similar. While a desktop Celeron is limited to only 66 MHz front side bus, the mobile Celeron at BGA-2 and Micro-PGA2 is running with 100 MHz front side bus.
This clearly shows that the limitation of the desktop Celeron to 66 MHz is not related to any technical difficulties.
Why is the Micro-PGA2 CPU may favorite? It's easily upgradeable!