Why is “disabled“ preferred over “handicapped”?

Solution 1:

According to linguist John McWhorter the answer is indeed history. Older terms accumulate baggage and are replaced by new terms in what is called the Euphemism Treadmill:

Crippled began as a sympathetic term. However, a sad reality of human society is that there are negative associations and even dismissal harboured against those with disabilities. Thus crippled became accreted with those overtones, so to speak, to the point that handicapped was fashioned as a replacement term free from such baggage.

However, because humans stayed human, it was impossible that handicapped would not, over time, become accreted with similar gunk. Enter disabled, which is now long-lived enough that many process it, too, as harbouring shades of abuse, which conditions a replacement such as differently abled.
— Euphemise this: McWhorter on The Euphemism Treadmill

Solution 2:

The two terms most commonly used to describe a person who has a limitation are "handicapped" and "disabled."

A disability is the result of a medically definable condition that limits a person's movements, senses, or activities.

A handicap is a barrier or circumstance that makes progress or success difficult, such as an impassable flight of stairs or a negative attitude toward a person who has a disability.

• A practical example: Janet Zeller, who has quadriplegia (some level of paralysis in all four limbs), has been told that she doesn't look "handicapped" when she is out paddling her sea kayak. Think about the situation. When Janet is paddling her sea kayak she is part of a sleek craft gliding through the water. There are no barriers to stop her or to "handicap" her. But she still has a disability.

The correct term is "disability"—a person with a disability. Person-first terminology is used because the person is more important than his or her disability.

(www.fs.fed.us)

Solution 3:

In Italian, "handicappato" has also been replaced with "disabile", the former was used as an insult to challenge someone's intelligence or behaviour. When I was a child living in the UK, the term handicapped was practically synonymous with "cretin" "stupid" "imbecile" "spastic" and "idiot" four of which, funnily enough, were terms that were originally used in the medical field.

Calling someone handicapped when they have no physical or mental disability is clearly a misuse, and calling someone handicapped even if they are paraplegic, hearing impaired or have learning difficulties is also rude. So, why have medical conditions been used as insults? Because people the world over are generally unhappy cruel creatures, and name calling requires no analysis, forethought and, ironically, no intelligence.

Solution 4:

The term "handicapped" is stronger than the term "disabled."

Both refer to a lack of ability, but while disabled means

not having one or more of the physical or mental abilities that most people have:

handicapped means

Having a condition that markedly restricts one's ability to function physically, mentally, or socially.

A disability may affect a person's life not at all, depending on what that person chooses to do, but it will arise to the level of handicap only if it does affect the life.

Consequently some people prefer the term "disabled" because it is less severe.