1. The original post mentions that the musicians' union, the American Federation of Musicians, lobbied for this for years.

2. These items take up more than the usually allowed amount of cabin baggage space. With high flight loads these days and a high proportion of people carrying on their bags instead of checking, passengers often have to gate-check baggage. It's inconvenient at best, and at worst it results in people losing their possessions.

This measure also decreases the revenue of airlines which they will either make up by increasing prices for everyone, reducing expenditure elsewhere, or lowering profits. The latter two don't affect me that much but you might feel differently if you were an employee or shareholder of the business.