I have a wooden raise garden, 4 foot by 8 foot by 24 inches in depth, containing 20 inches of rich soil. The bottom of the raised garden is a foot off the gound. Last spring I introduced into the garden 750 night crawlers I purchased from Uncle Jim's worm farm. I grew tomatoes, squash, onions, radishes and pickles. We left some of the dead leaves on the surface, as well as poked some of them into the soil for the worms to feed on. There are very small holes for drainage in the bottom of the box. I also had a 30% shad netting over the garden to keep the birds out. By the end of fall, all of the worms had disappeared, and I have absolutely no idea why. I have asked the people at Uncle Jim's worm farm the question, as well as other sources and have yet to come up with an answer. I know night crawlers come to the top to feed at night, but like undisturbed soil to burrow in, and the bottom eight to ten inches of soil was not disturbed. Does anyone have any idea what may have happened to the worms?
Hope I can help with this mystery. 'Rich' soil means what in your opinion? Did you have a soil test? Worms need DECOMPOSED ORGANIC MATTER on which to feed. Not live stuff. The decomposers have to decompose the live stuff before that material is edible for nightcrawlers, or earthworms.
Did you use garden soil for this raised bed?
Did you apply any decomposed organic matter at all? These worms HAVE TO HAVE decomposed organic matter or they starve. If you use any bark or chips or anything that was once alive and now dead but NOT decomposed those worms have nothing to eat. Those worm people should know this, how weird.
Next time all you have to do is find DECOMPOSED ORGANIC MATTER, purchase by the bale. Usually not completely decomposed but at least it doesn't come with weed seeds and pesticide residues...which might also be an avenue to consider. Are these beds made with pressure treated wood? That would also kill micro and macro organisms plants need to up take chemicals for photosynthesis.
What else have you added to your soil? Raised beds do not need any structural assist by wood or concrete. To leave your beds ON THE GROUND to drain and create an ecosystem with the sublayers of soil is critical. That is an awful lot of worms with no food and no place to go to have their own families. Grins, but this is kinda sad. Nightcrawlers do not like to burrow into sub soil that is compact. They prefer some tilth for air. They are wonderful for coming up to the surface to eat DECOMPOSED ORGANIC MATTER not UNDECOMPOSED OR FRESH LEAVES...then going back into the soil to poop it out and mix organic material into your soil for you. THAT is the beauty of these macro soil organisms. But you have to FEED them DECOMPOSED organic matter. I buy it by the bale. Don't go to your land fill unless they are able to give you a soil test sheet that tells you there are no pesticide residues in the mulch. Make your own compost or DECOMPOSED organic matter but good luck in making enough of it to top dress your beds at least 2 inches thick and doing this at least a second time preferably a third time during the growing season. Why did you raise your beds above ground? We can discuss further if you send more information about your construction, what soil you have used and what all you've added. Poor worms. But this is how we learn...gardening. Otherwise, worms do not do anything for your crops, not in those numbers certainly. If you supply the food THEY WILL COME to your restaurant!! If your garden soil is part of the bigger picture!
I'm not sure why you put 750 night crawlers into your raised bed - these worms are usually purchased for fishing bait, or to put in a composting pile or bin. I imagine you thought they might increase the fertility of the soil in your raised bed, but if conditions are suitable, raised off the ground or not, worms will arrive (usually earthworms rather than brandlings, red wrigglers or night crawlers) - but only if they find the conditions amenable.
If you used African night crawlers, they don't like cold temperatures, so would likely have gone before winter, even if they hadn't already departed. Ultimately, there is no point in adding worms to an area such as this, because, if conditions are right, worms will be there, if they're not, they won't. If you live somewhere cold in winter, even earthworms burrow deep down in to the soil to get through the cold temperatures, and only become active nearer the surface once the soil has warmed again. Two feet deep, suspended in the air, likely means no worms will stay all year anyway, unless you don't have cold winters.