Complex sentence: The local taxation rate in Kauniainen is the lowest in Finland (%), and so it tends to attract the highest earners, hence it is a generally wealthy place, and can thus keep the taxation rate low without compromising the service to the inhabitants.
Split into simpler sentences: The local taxation rate in Kauniainen is the lowest in Finland (%), which makes the city attractive to the highest earners. This in turn makes the average income generally high, making it possible to keep the taxation rate low without compromising the service to the inhabitants.