The single most important reason, is their multi-party system. Pretty much the same all across Europe.
Each country will usually have two or more "major" parties, from which the leadership will come. Those major parties will cover the center.
Secondary parties usually are at the fringes, and stand little to no chance to win. Their best chance is to be included in a coalition.
Typically those "fringe" players (parties, individuals) will include some policies that are portrayed as "extreme." Some of the fringe parties' candidates are kooks, too.
Finally with WWII in the fairly recent past, Europeans tend towards their "center." And Europeans also tend to like their mixed-economies.
