Why is the term "lemon" often used when talking about something that is defective or has poor quality (mainly a car)?

Great question.
lemon "worthless thing," 1909, Amer.Eng. slang; from lemon (1), perhaps via criminal slang sense of "a person who is a loser, a simpleton," which is perhaps from the notion of someone a sharper can "suck the juice out of." A pool hall hustle was called a lemon game (1908); while to hand someone a lemon was British slang (1906) for "to pass off a sub-standard article as a good one." Or it simply may be a metaphor for something which "leaves a bad taste in one's mouth."
http://www.etymonline.com/index.php?search=lemon&searchmode=none

The Citroen 2CV was an ugly, slow French-made car from the '40s. Since "lemon" in french is "citron", I figured it was just way too much of a coincidence.

because by the term "you bought a lemon (junk car)" it means you got a SOUR deal

The history of the lemon law starts in 1979 with a lady named Rosemary Shahan from Lemon Grove, California. She had bought a lemon that still wasn’t working after waiting 3 months to get it repaired. She then picketed the mechanics for 5 months which created a public outcry against the treatment customers received from mechanics and car dealers. This public outcry became the basis for a citizen movement for lemon laws which formed Consumers for Auto Reliability and Safety (CARS) an organization that made Rosemary Shahan their head. The first states to introduce Lemon laws were Connecticut and California. They introduced lemons laws 1982.

http://ezinearticles.com/?Lemon-Law-101&id=112697

http://www.valvoline.com/carcare/articleviewer.asp?pg=dsm20010801l1&cccid=3&scccid=5

1. A tasty fruit that makes you make a funny face.

2. A fan fiction that is based on an anime/manga or video game that is sexualy oriented.

3. Something that was useless or crappy.

4. A lame person or thing.

5. A bitter fruit.

6. A car that you buy that you think is good, but really it sucks like hell and will break right after you buy it.

7. an object of dubious value; a piece of crap.

(many more...)

http://www.urbandictionary.com/define.php?term=lemon

a motor vehicle with many mechanical problems.

http://onlineslangdictionary.com/definition+of/a+lemon

The term 'lemon' to describe a highly flawed item predates its use in describing cars.

http://en.wikipedia.org/wiki/Lemon_(automobile)

In American slang, a "lemon" is a car that causes more trouble that it is worth, as in "I bought a car from that dealership, but it turned out to be a lemon."

In Australian slang, a "lemon" is a lesbian (originally just a sour woman), as in "This town seems full of lemons."

In Cockney rhyming slang, a "lemon" is a favor -" lemon flavour" - as in "Gi' us a lemon, mate."

In fan fiction, a 'lemon' is a scene or story that includes explicitly described sex.

My guess...a lemon is a very sour thing...a bad car sours the owner...nothing sweet about it. :)