Why were the impressionist painters not popular during their time?

Because immediately before them painting was done in a 'photographic' manner, because cameras had not been invented.  So visual records had to be very precise. 

Look at the French painter 'Ingres' or 'David', visually everything is there, that a camera would catch.  With the invention of cameras that need was removed and painters could paint more emotionally. 

Nobody really cared about paintings nor do they now.

Two Anonymouses quite wrong. Realistic painting require much more skill than Impressionism. Paul Durand Ruel was con man. Impressionism not make any sense. Is quite stupid concept. 

They weren't unpopular. They sold pretty well, but a lot of snobbish critics didn't like impressionism. Critical taste finally caught up with popular taste.

Because their style was so very different from the accepted style of painting at the time.

They should never have been popular, since their painting is immature. Spanish Surrealist Salvador Dali said that Impressionism is a poor solution to a non-existent problem, and that makes sense. Pointillism is the ultimate conclusion of Impressionism. People were accustomed to much more mature paintings, so they ridiculed Impressionism, as it richly deserves. Some later painters were influenced by it. The main Post-Impressionists were Vincent van Gogh   who pioneered Expressionism; Paul Gaugin pioneer of Primitivism and Paul Cezanne whose works inspired Cubism. Salvador Dali said that Paul Cezanne could have painted as well with his feet as his hands. We are in a Dark Age of Art. That began with Impressionism.

Part of it was the same reason that Jazz, Rock and Rap were all unpopular among mainstream and "cultured" musical fans in their early years.  Arts are a reflection of cultural values and just as there are popular notions of how music "should" sound, there are commonly held beliefs on how visual arts should appear.

Many felt painters had an obligation to present "real" images.  With photography in its infancy and distant travel still something reserved for a select few, the only view of distant lands and sites for many were the images they would see in museums and public exhibitions.  Stylized representations were seen as "frivolous" and indulgent, even insulting to the "refined eye."

Imagine you were unable to attend a historic event and were cut off from all media.  A friend of yours is traveling to that event so you give him a HD video camera and ask him to record it for you.  When he returns, you find the video he took is a series of weird vignettes and blurry images.  When you ask "what happened?" he gives you a bunch of metaphors and and his feelings about the event, but no specifics.

That is the level of frustration many art aficionados experienced with impressionism...not only that the artist was wasting time and resources but also failing in a responsibility.

Critics disliked them. But the art-buying public guided by enlightened dealers like Paul Durand-Ruel in London saw their potential. Russian collectors were keen - French Impressionism is well-represented in The Hermiage.

BECAUSE REALISM WAS POPULAR.INSTEAD. That's what people were buying and it was unfortunately thought to require greater skill as a painter. .  

They had bad breath