I wonder why make us/eu versions
Because there are different frequencies used in europe GSM900, GSM1800 and in america, asia et cetera GSM8500, GSM1900, tansania GSM400, not used: GSM700, not to mention the difference between GSM, UMTS/WCDMA and CDMA (where CDMA2000 does not require a SIM, also CDMA2000 is incompatible to GSM/UMTS stuff and vice versa).
UMTS and so HSPA-frequencies are alot more - yet 700, 750, 800, 850, 900, 1500, 1700, 1800, 1900, 2100, 2600 MHz bands. So antennas, radio roms and settings must be set according to the standards used in every country. Quadband GSM and quadband UMTS are quite handy, former devices were quad/triband, but I don't think there will ever be a device that's ready for all.