It seems to have had a short life if it is to be discontinued but maybe the understated looks coupled with a very high price are the reasons for its demise. Perhaps people spending that kind of money on a watch who aren't avid G-Shock collectors fail to appreciate its qualities and want more bling for their buck; I am always attracted to dials and whizzing hands and the rest. Or the GW-M5600-1ER is available with solar and atomic functionality for about one third of the price though it looks rather more fussy in its decoration and labeling.
Well done on securing one of the last ones, enjoy it!
Just edited to add: Calculations have just been thrown out of whack through reading
this post about a good deal on the GW-M5600,
six for the price of the visually superior GW-5000. Almost cheap enough to tempt me to try and love the "square" G-Shock.