Chrons was names after Dr. Burril Chron who described it in the 30s, Parkinson's was again after the person to describe it Dr. James Parkinson in the 1800s, ALS is Lou Gehrig's because of baseball player Lou Gehrig who was a very well known person who had it.
All these are Googleable btw
most of those diseases are named after the person that discovered, definitively categorized, or first described the disease. others are named after a famous patient, or the first known patient. It usually comes from diseases that aren't communicable, and thus weren't named before systematic medical or scientific research into them.
Parkinson's for example was commonly called "shaking palsy". James Parkinson wrote "an essay on the shaking palsy" in 1817. I believe it was first described in western medicine by Galen in the early AD times, but it had other names in other areas before then. there's plenty of other eponymous diseases of course.
Huntington's is one that was named after a patient, though the patient was also a doctor. he diagnosed it in himself, his father, and grandfather. as far as I'm aware, the majority of the autoeponymous (self named basically) diseases were also similar in that the person who had the disease also was the first to quantify it as a disease.
edit:I noticed you asked for the why/how. The how is because of the above. The why though isn't.
with diseases, particularly those named and/or discovered in the last two hundred years, there's typically the habit of giving them accurate names. ALS is short for amyotrophic lateral sclerosis. if you learn medical terminology, that means something. It is very descriptive of the nature of the illness. But it doesn't communicate very well to someone without that knowledge. that's why common names come about. ALS is also known as Lou gherig's disease because he was famous and had it. It gives the layman an easier name to pronounce and remember.
there's been a move to just using acronyms for a while like ALS, but the eponymous ones tend to keep the name because they're already recognized under that name by everyone.