Fundamental rights are enshrined in Part III of the constitution and are enforceable by court. While DPSP are enshrined in part IV of the constitution and are not enforceable. Both are complimentary to each other. As Fundamental rights ensures political democracy which is hollow if not accompanied by
social and economic democracy.
The reasons for according such differential status was the inadequacy of capital and resources to provide the social and economic benefits when constitution was started. For example education and health upgradation need investment, but british policies have rendered Indian exchequer poor.
Second, Major threats faced by India was balkanisation and communalism. Enforcement of Uniform civil code would have been a difficult task at that point of time.
DPSP are vague in nature with no concrete guidelines. The implementation with uniform policies would have undermined the elected state government and hence the federal structure.