I didn't even know it was a ship when I first baught it! I just remember a big reason why I bought Overwatch was Mccree himself. Anybody with a BAMF buckle has my respect. I ended up finding out about about the ship from a friend, and how the name "Mchanzo" tickled my ears.
From the get go, a big reason why people ship Mchanzo is because of the massive contrast of two cultures. Samurai and Cowboys had rather different values, yet were adored for their power and wonderful stories that they told to other people about their many adventures.
Even though both Hanzo and Mccree both have not had any interactions in the game, (yet) it is still entertaining to construe situations where their cultures and personalities clash, and then everything working out in the end. One could also compare them to two sides of the same coin or Yin and Yang, depending on your perspective. I hope you all enjoyed my fanart!