Because of their color... I think usually fire hydrants have only one color (normally Red in Japan) but these hydrants in Otaru are colorful.
They have other colors too!!
I'll tell you the reason.
There are lots of slopes in Otaru city include heavy slopes.
If a fire occurs and put out the fire using only one area's hydrants, Otaru's source will lose the balance and it will be impossible to supply water stably. That's why, these colors distinguish supply area of fire-fighting water.
It might be interesting to look for colorful hydrants in Otaru!
This sign shows the angle of slope, this is not rare in Otaru