In keeping with the SDMB tradition of asking such questions right before Shabbes begins:
Why are Mogen David and Manischewitz Kosher wines so darn sweet?
It’s basically a cultural thing.
But I think it may have something to do with children.
Kosher wines are usually drunk at the family dinner table, with children. But , hey, everybody knows that you don’t give alcohol to kids, right? So let’s use the sickly sweet stuff that tastes like cough syrup. That’s not the same as giving kids champagne.
and by the way, its not just Shabbes, its also the start of Passover . So we won’t be hearing from zev until Tuesday 
I for one freakin’ LOVE Manischewitz! I regret that the Blackberry Cream Sherry has been replaced with just Blackberry Wine.
Another excellent combination of reply and user name.
I don’t like real (as in non-Concord) sweet wines much. The Manichewitz – the one sip I had of it – was horrific. Fortunately I was then able to switch to the regular grape juice (this was at a seder, and they had both available), which I am not a big fan of, but was at least drinkable.
For whatever reason, I’ve found alcohol stashed away somewhere in every house I’ve bought.
The house I own now? A half drunken bottle of Manischewitz. I’m Jewish, but I still poured the remainder down the sink.