Short positions are much different than long positions. Building an exchange for long positions is simple. User A wants to buy for price P, User B wants to sell for price P. Done.

Short selling in currencies is done in one of two ways, both of which are more complicated. For normal securities like stocks, short selling is done by loaning the security. For obvious reasons, loans between anonymous cryptocurrency users is problematic. Bitfinex is one of the only exchanges supporting lending features, but it's complicated and and doesn't support STEEM. You can short BTC, but you need to borrow BTC, convert it to USD, wait some time, and then convert it back, returning the borrowed sum in BTC + interest.

The other way of shorting a currency is via a currency pair. Which involves the implicit buying and selling of currencies as the pairs are traded. Again, way more complicated then a simple long position.