Although 3rd party developers eventually brought ICS (and Jelly Bean for that matter) to the Nexus One with a lot of hacking, there were good technical reasons why there was never an official release. ICS needed drastically more space on NAND than Gingerbread, as evidence by 3rd party ICS ROMs requiring you to repartition your NAND as well as mount /data on the SD card if you wanted to install more than two or three apps, which then led to greatly reduced I/O performance.