|Because those are all the LGA 775 processors that were made.|
The best you can do is a Pentium 4 Extreme Edition 3.4GHz.
Anything above a Pentium 4 will not work with the 915 motherboard.
The processors that won't work concludes the following....
Pentium D (Old Dual Cores)
Core 2 Duo (All Series)
Pentuim D E series (Based on the Core 2 Duo)
Core 2 Quad (All Series)
Celeron E series (Based on Core 2 Duo)
You're best bet would be to go to ebay and find some Pentuim 4's for sale cheap. Here you can find some LGA775 Pentuim 4's really cheap.
Iron Sharpens Iron.