The reason is very simple: because the government spend a huge amount of money on useless things and there's also a lot of corruption and fraud. 1. They make buildings worth millions in small towns (Google for example "Gare Mons" or "Gare Liege").
For residents of Belgium, communal taxes are levied at rates varying from 0% to 9% of the income tax due. The average rate being 7%. For non-residents, a flat surcharge of 7% is due. In some cases, communal taxes may also be levied on exempted foreign-source income.