why do some countries ask for a visa and some dont?

Answer 1

Visa requirements vary country by country, often linked to reciprocal international agreements designed to provide for the free-flow of legitimate travelers.

See the following link for information on the US Visa Waiver Program (VWP), as an example. VWP enables nationals of certain countries to travel to the United States for tourism or business for stays of 90 days or less without obtaining a visa, with the objective of eliminating unnecessary barriers to travel, stimulating the tourism industry, and permitting the Department of State to re-focus certain consular resources.

http://travel.state.gov/visa/temp/without/without_1990.html#vwp

Answer 2

Well developed countries require visas into order to prevent people to come to their country without any valid purpose while poor countries do not require visas in order to encourage tourists to come to their country.