To understand why, we need to look at the differences between these two languages. Both Mandarin and Cantonese use Chinese Characters, and fall under the same family of languages.
What makes learning Mandarin substantially easier for Cantonese speakers is the fact that most words between the two languages are the same. Additionally, most of the grammar between the two languages is also the same.
For instance, let's look at this sentence (I want to go out), and compare Cantonese vs Mandarin: