There are multiple reasons:
- Minimum limit of buying SP directly from (https://blocktrades.us/) = 37.0705 BTC
- If you buy from exchanges, you can buy cheaper and in bulk (If you check now if you buy 1 BTC = 1266.649 SP in (https://blocktrades.us/) while on Poloniex you can buy 1 BTC = 1305.824 SP