When you say 'retains hardly any moisture,' I picture that within a day after watering the surface is crumbly dry and if you push your finger down a couple inches it's, well, cool but definitely not wet. If these assumptions are wrong, so will be this reply.
It sounds like the mix isn't the issue. Your soil this year sounds top-notch. It also may well be not much different from what you bought last year. When you water amply, the soil naturally drains to the point that, picking a rather arbitrary number from a wide range, 10% of its depth is water - say 2-3" depth equivalent. See http://www.fao.org/docrep/r4082e/r4082e03.htm for a good if long description if you are curious. The drainage is a good thing. Soil has and needs lots of air pores as well as room for the dirt particles. With the excellent drainage you provide and good loose growing soil, this will happen within a couple hours after watering, or sooner than if the garden were at grade.
The water goes away quickly. Vegetables drink heavily, many of them > 1" / week. The surface dries out quickly from sun, and most vegetables are grown in full sun. In my dry climate, I irrigate my vegetables every day. In midsummer if I watered less often than every other day given no rain, it would hamper the plants' growth.
The more clay in the soil, the more water it will retain. But high clay content in most other ways is not an optimal growing medium. Still, if you want more water retention, and depending on the texture of your native soil, you may be able to use more dirt and less compost.