Final answer:
The land grants in East and Central Texas were issued to promote agriculture in the fertile soil, which was also ideal for expanding slavery and the cotton economy, attracting many American settlers.
Step-by-step explanation:
The Spanish and Mexican governments issued land grants mostly in East and Central Texas during the colonial period primarily because of the fertile soil which was suitable for agriculture. This region's soil and climate were conducive to expanding slavery and the cotton kingdom, which were of great interest to many Americans, particularly from slave states. Land in East and Central Texas was offered at generous terms, including low prices per acre, the ability to pay in installments, no minimum purchase requirement, and tax exemptions, making it an attractive option for settlers.