So far as I know, password boxes and PINs are always obscured in some way in order to prevent people from looking over your shoulder when you enter it. However, other important information that I type into a web form (credit card number, social security number, etc.) that I would also like to protect from rubbernecking is hardly ever obscured. 

As a concrete example typing your login password into Steam or Amazon is properly blanked out but typing in your credit card number when making a purchase is revealed as plaintext. Intuitively, it feels like you would want to blank anything that could be stolen, credit card info included.

Is there any particular reason why only passwords and no other sensitive data is usually obscured?

I don't think this is about secret vs sensitive information, eg "secret information is masked and sensitive isn't." (the distinction is problematic; many secrets need to be shared too).

I think cc and ssn information isn't blanked for several reasons. They're not hard fact, but still, here are a few:


Doing what everybody else is doing, not surprising the user. Passwords came long before web forms and were already masked. So when they were used in login forms, they were masked too. When online business and administration came along, prior art (paper forms) required unmasked entry of sensitive information (obviously), so web forms made the same choice.
It would be very easy to make a mistake and enter a wrong number if you didn't see what you typed. In the case of a wrong password, this is only mildly annoying and you get instant feedback; in the case of other wrong sensitive data, this might have further consequences which isn't immediately clear (such as a request to a government office being denied because of a wrong SSN or a payment not going through - although in the cc and ssn cases, this is usually  hedged against by looking if the checksum is right and providing immediate feedback if it wasn't)
Many people don't know their cc number from memory and copy it down from the actual credit card. So the number is there to be seen by a bystander anyway; blanking the entry field wouldn't help much to protect the number at the client end and thus doesn't merit the additional difficulties caused by 2.
Impact - A stolen cc number doesn't necessarily have the same impact as a stolen password, especially if you consider that information stolen by looking over your shoulder would most likely be stolen by people who know you (collegues at work, family members, classmates etc) and want your password for a very specific reason that would have personal consequences. OTOH, even if they were interested in your cc number, you wouldn't have to take the financial damage because the cc company would probably cover it. For the same reason, you don't mind letting a waiter or store clerk see your credit card, even though he could copy it and sell it to someone else...


Number 4 may not apply to other kinds of sensitive information, though.
