The reason behind that is that DoCoMo uses a 2G wireless technology that's pretty much incompatible with anything else in the rest of the world. In other words, they won't loose a lot of roaming charges with this move.
By switching-off their 2G network they'll significantly save money on two fronts: First, there's one network layer less to to keep running so that certainly saves a great deal of money. Second, they no longer need proprietary dual mode 2G/3G devices and can go forward with dual mode GSM/UMTS mobiles that are sold in the rest of the world (+ an additional frequency band, see below) or triple mode GSM/UMTS/LTE devices. And while we are at it, does anyone know if current devices are still dual mode or has DoCoMo phased this out already and just keeps the 2G network running for legacy devices?
And a final thought on this one for today: Looks like in Japan DoCoMo doesn't only use the 2.1 GHz band for 3G but also a band in the 800 MHz region (FOMA Plus) which is also used by their 2G PDC system. Here's some more details on this from a report on a Blackberry version for Japan. That report also indicates that the band is different from the 850 MHz band used in the Americas. It would be too simple otherwise... So once they switch off their 2G system, they can do further re-farming. Also a nice benefit.