1. It's rare, so not many programmers have the pattern matching built up to read it easily.
2. Chained if/else statements have the benefit of indentations helping to show structure. The moment you add newlines to help show structure, a chain of ?:?:?:'s becomes much easier to read.
Most ?: expressions I see are uses inline, such as foo(a ? b : c). When you do that, you sometimes have to mentally unwind "ok so if a... what's a? why a? ok, so if a, then foo(b), else foo(c)". Putting the if(a) up front means you're already thinking about it by the time you get to the function call. So I reserve using ?: for when the difference between the two outcomes is minimal.