I think Grace Zabriskie is an amazing actress, and I wish that there was more of Sarah Palmer in Twin Peaks.  Her scenes are some of the best in the entire series!  One thing I always wondered... why wasn't her name in the opening credits?  In Season 1, she was featured in nearly every episode and she was obviously one of the most important characters.  Come to think of it, there are several important actors who aren't listed in the opening credits.  Wendy Robie, Eric DaRe & Harry Goaz are a few that come to mind.  Why do you think some actors weren't listed in the opening credits, even though they were featured in nearly every episode?

Kimmy Robertson's name was added to the opening credits for the second season...

One other thing... in the Palmer living room there is a huge pot of dried reeds that is the exact color, shape and texture of Grace Zabriskie's afro.  Love it!    

it's a contractual thing.  When Grace signed on, it was probably discussed in what kind of capacity.  (though funny Ray Wise had a different type of billing.)  She was most likely to be used only as a guest star.

In developing it, she was not considered to be a series regular -even if coincidentally she kept reappearing.  And sure enough after the Palmer saga, she disappears.

The actors in the opening titles were probably contracted to have to appear in so many episodes as is dictated in their contracts.  Kimmy Robertson's was renegotiated during the summer of 1990. 

This is different than James Marshall and Joan Chen who I believe both left the show (I know Marshall did, don't remember if Chen left or like Wise learned she was being killed off).

And of course, some actors had special credits, which is typically negotiated by agents and lawyers -ie.  McLachlan and Otkean get the leadins -while the rest are alphabetical.  Chen and Laurie -the two biggest stars at the time get special credits at the end of the opening montage.

Her name is in the opening credits. Eric DaRe and Harry Goaz are there too.

Those names appear after the title sequence. Grace's name is under the "Guest Starring" title.