I think it's mostly because it's so long and somewhat convoluted. "Благодаря" has 4 syllables while "мерси" only has 2. It's similar to saying "thanks" instead

I think it's mostly because it's so long and somewhat convoluted. "Благодаря" has 4 syllables while "мерси" only has 2. It's similar to saying "thanks" instead of "thank you" but that comparison is lost because the two words in Bulgarian are so different.