It is difficult to eliminate pork barrel spending for two main reasons. First, many people have strong incentives to support pork barrel spending. Second, essentially no one has strong incentives to oppose it.
Pork barrel spending brings real benefits to various people. The representatives who win pork for their districts improve their standing in the eyes of their constituents and become more likely to win reelection. Companies and individuals in the districts that get the pork benefit economically. A pork barrel project creates jobs and otherwise helps the economy of the various congressional districts. Therefore, there is a great deal of incentive to support such spending.
By contrast, no one really benefits significantly from the killing of any one pork barrel project. Of course, ending pork would cut government spending and ease the burden on tax payers to some degree. But no individual program is expensive enough to impact tax payers. Therefore, there is no real outcry over pork projects because we do not feel the impact from them in the same way that we might feel a tax increase.
When some people gain from a policy and no one is badly harmed, the policy tends to remain popular. That is why pork is hard to eradicate.