The reason the service sector contributes the highest to India's Gross Domestic Product (GDP) post-1991 can be attributed to several factors:

1. Economic Reforms: Post-1991, India initiated economic reforms and liberalization policies, which led to increased privatization, globalization, and a more open economy. This allowed the service sector to flourish, as it is less capital-intensive and more adaptable to these changes.

2. Information Technology (IT) Boom: India's IT and software services industry experienced tremendous growth during this period, leading to increased exports of IT services, business process outsourcing (BPO), and software development. This played a significant role in boosting the service sector's contribution to the GDP.

3. Skill and Education: India has a large pool of skilled professionals in fields such as information technology, finance, healthcare, and education, which has led to a growth in service-related industries.

4. Consumption Growth: With the rise in income levels and urbanization, there has been an increased demand for services like banking, insurance, healthcare, education, and entertainment.

5. Export of Services: India's service sector has been active in exporting services to other countries, earning foreign exchange and contributing to the GDP.

As a result of these factors, the service sector has become the largest contributor to India's GDP post-1991, surpassing the manufacturing and primary sectors.