Why were changes made from the book?

[eggplant107 at hotmail.com]

No: HPFGUIDX 29709

In the book the species of snake is native to Brazil, in the movie it 
comes from Burma. In the book Hagrid got Fluffy from a Greek fellow 
(Greek mythology has a 3 headed dog)but in the movie Hagrid got 
Fluffy from an Irishman. Any ideas for why the changes were made?