Can someone list a few popular constructed languages (maybe comparing them to programming languages)? I'd only heard of Lojban and Esperanto before reading this.
Try http://www.reddit.com/r/conlangs for discussion and lists of languages. There are many subreddits for specific languages. For example, Toki Pona [1] and Lojban [2].
Regardless of the point, the truth is Esperanto's lexicon is mostly inspired by Romance languages whereas Slovio's lexicon is mostly inspired by Slavic languages. It's not that shocking of a comparison.
[1] http://www.reddit.com/r/tokipona [2] http://www.reddit.com/r/lojban