One reason: Most RDBMS's aren't designed for "waiting for newly queued items". You literally have to have each worker poll at some time interval for new items: `SELECT * FROM jobs WHERE status = 'new'`, transactionally claim the job, and if a worker dies, un-claim it. It's not that it's not possible to use an RDBMS and work around these problems, but generally, it won't lead to the most scalable and robust solution for the problem. Message brokers / AMQP are better designed for this problem set. If you have a small site, with a low volume queue, your RDBMS might be just fine though.