The reasons are, first, that poverty and deprivation are not particularly concentrated. Yes, there are some places much worse than others, but any policy to target the areas most affected will always miss most of the target group. Most poor people do not live in the small number of "worst" target areas.
Second, these are human problems, not problems of place. People move around. One estate or area may improve, but only at the cost of the deprivation, and often the same deprived people, moving somewhere else - unless there is action to reduce the net amount of poverty and deprivation.
The previous government commissioned, and then ignored, a review of urban policy led by Professor Robson at Manchester University. It showed that 30 years of area-targeted initiatives have failed to improve the areas concerned relative to the country as a whole. The report commented that targeting people rather than areas might be more appropriate.
I well remember working on economic development in Peckham when the then government announced a task force to "swoop" on the area with pounds 2m to spend. Local views were divided between those who thought that the money would not go far when divided equally between local people, and those who said that it was just not enough to knock the place down. People in deprived areas are deeply cynical of these government initiatives, since they know that what is really necessary to tackle poverty is a matter of action on low wages, high unemployment, low benefits, and a regressive tax system.
If Mr Brown wants to tackle poverty, then he should stay in the Treasury working on the tax system, and stop swooping on estates, which will only alarm the residents.