The history of the 25nm sector altitude is that it was originally devised for aircraft in an emergency situation. Pilots recognised early on that it was also had other uses - arrival planning is an obvious example - and nowadays it is used primarily as an orientation/planning aid. You get a minimum 300m (1000ft) terrain separation in each sector (up to 600m, 2000ft in mountainous terrain).
I think Europe is too small to accommodate 100nm sector altitude charts!