What that does is tell Google (or any search bot) not to look at any files that are in any of those folders. I’m not all that familiar with Wordpress, but I’m guessing that at least some important files will be included in those folders, so if you’re telling Googlebot not to look at them then it’s no wonder that it has dropped you from its listing.
As a general rule, it’s very rare that you need to disallow anything in robots.txt. First, because most of the time it’s in your interest to allow search engine spiders to wander all over your site, going wherever they please, looking at everything - second, because it’s absolutely useless as a security measure; if you’re doing it to prevent bots from accessing secure or hidden areas of your site then forget it, they don’t have to obey robots.txt, and as soon as you tell them not to go there, any malicious bot will go there straight away!