Actually Bangkok and a fairly large central region (Central and East on the map) is not very poor - its per capita GDP is not far from the Taiwan/Singapore level.
Oh please, GDP per capita is no way to measure poverty. Bangkok has a certain amount of very, very rich people and sucks in most of the wealth from the rest of the country without giving much back in return. And at the same time it has some big, big slums and major problems with all kinds of things that are generally regarded as indicators of deprivation.