There are a couple practical reasons. 2 Corinthians is very personal in nature, and it is sort of a one-sided conversation. You could look at the commentary introductions to get a technical description of the genre, but I just want to note that it is very similar to a type of Greek that we are the least familiar with — personal conversation.
I was thinking about this today, and my mind wandered in a less practical direction. Think about the nature of the conflicts in 2 Corinthians. It’s personal conflicts about money and slander and trust a whole host of people issues.
2 Corinthians is about the hardest issues in life, and so it is somewhat fitting that the Greek is the toughest to piece together.