Shortest summary I could quickly find:

> "...building a suspension bridge of this scale poses significant engineering challenges. The Strait of Messina is known for strong winds, seismic activity, and deep waters, all of which complicate construction and long-term stability. Engineers will need to ensure the structure can withstand earthquakes, which aren't unheard of in the region, while addressing corrosion from the salty marine environment."

-- https://www.iflscience.com/worlds-longest-suspension-bridge-...

I remember reading an article, posted here on HN, that went into much more depth about why this was all unusually challenging, but I haven't found it again.