The answer is high cost-performance,
because the Philippines has advantages in the following aspects;
Philippines uses not only Tagalog but also English as an official language. People there widely use English in education, business and mass communication. Due to these facts, their English level is very high. For the Business English Index (commonly called BEI which is used to evaluate non-native business English ability), Filipinos have the top score; the highest in the world. You will discover that most ESL teachers can speak English as well as highly-educated English native speakers.
Cost of living is minimal in the Philippines. Food, accommodation, spas , beauty parlors, bars, restaurants, etc.....are very much affordable. Labor cost is especially very low. That is one of the reasons we can learn English for a minimum price. When we compare it with the cost of learning English in the US or UK, one third or one forth the expense is attractive.
Filipinos are very talented and eager to teach foreign students, because they are full of hospitality. They are almost always cheerful, positive, generous and patient in teaching. So even beginner students can try to speak English to them without much hesitation, especially during one-on-one lessons. This means you can enjoy lessons and make great progress without even noticing it!