Interesting! I can recall Ullasa Paravaigal, 47 Natkal being filmed in Europe, both are filmed around 1980-81. I remember 80s had a flurry of movies that were filmed in Singapore and Malaysia: Ninaithale Inikkum, Priya, Thai Mookambigai (first few mins), Ooru Vittu Ooru Vandhu, Enakkul Oruvan.
I'd guess the reasons would be lot of Tamil diaspora settlement which would also mean availability of local manpower and resources, ease of permissions etc. Thats why you'd still see lots of Canada, Switzerland,Malaysia being filmed.