When you earn money , or generate wealth, you are essentially converting your limitless potential into something tangible , realised and therefore, limited.
So , in a sense, while you are gaining something valuable , money, that allows you to buy other things that sustain you , give you pleasure and happiness, you are also losing something. By converting your eternal potential into something material , you are tied to the mortal and not focussed on the immortal.
The other point is desire. Each and every action that we take is because we desire something.
The desire for salvation is also a type of desire. The only difference is that once you attain salvation , then there is no more karma generation. All your karma is wiped clean. So there is nothing more to desire after you have attained salvation.
The richer you are , the more attached you are to life. Consequently, the harder it is to let go when the time comes to realise your true nature.