Slimcoin had a little boom and I think to remember that at the very beginning it was one of the top-100 coins at Coinmarketcap. But then it was affected by a long decline which almost led into the "death" of the coin in late 2016 (it was gjhiggins who saved it actually, releasing a new version).

I think there were four major factors causing this decline:
- first, the original client was very buggy, so people left because of usability and security issues (frequent orphan blocks/forks, the client getting stuck randomly, etc). This is now fortunately solved.
- second, many people tried out the PoB feature, but were disappointed by the low returns  and left. You have to mint a long time to get profitable - in times of high burning activity you have to wait at least a year (like now)
- third, the original developer left the project, with major bugs staying unsolved.
- fourth, the fact that it was only traded on BTER and lost "everything" when BTER delisted it in early 2016.

Well, after this decline, Slimcoin stayed basically "under the radar" until now. And the crypto market has changed dramatically in 2016 and 2017 - there are now projects that have tens or hundreds of millions of marketing funds collected by ICOs, and decentralized projects like Slimcoin can hardly compete. Also, it doesn't help that exchanges charge immense fees now to add a coin.

There are, however, decentralized projects which were able to compete with the centralized competition (Monero, for example, and obviously Bitcoin which seems still "uncontested"). This gives me hope that Slimcoin could do much better in the future. I think the boom of centralized coins won't last forever. As people realize they're often scams or don't make much sense (why use a centralized coin if you have Paypal?) decentralized coins will return to become dominant.