The reason is more or less obvious. The site owner, PM, intended this site for the beginners, who are mostly very young kids. In fact, a fraction of our (newer) users are 10-13 years of age. While it is acceptable in most (Western) cultures to swear, some of the more conservative cultures don't approve of swearing. Besides, Pokemon was meant for kids to begin with, so there's no reason to bring in "adult" language.
That kids these days swear tons doesn't dethrone the fact that most of the content is extremely child-friendly, and it would make no sense to bring in bad words. May be, a way to look at it might be ad revenue. The more family friendly the site, the more advertisers want to use the space. This makes sense because parents sometimes tend to monitor online activity, and prevent children from accessing certain sites, reducing income generated. So from a business point of view, it makes sense to censor anything that might affect how "safe" a website is.