It seems quite unfair that this is the case, since when did sleeves become so passe? There are many reasons to choose a sleeved design. Lets take into consideration the weight of a bridal gown. Depending on the syle you choose, there could be 3 to 4 to 5+ layers of fabric in the bodice alone, not to mention the skirt which will undoubtably have atleast that many. If the dress is made in satin or heavy lace, this adds to the weight. A beaded design will add even a little more. Without straps to support the dress on your shoulders, the requirement is heavily boned underpinnings sewn into the dress, commonly referred to as boning. The dress will need to be anchored to your waist and kept upright by these underpinnings. The dress also needs to be fitted to you precisely and quite snug, so there is no gapping or slipping of the bodice.
I adore a strapless dress as much as anyone, but with these considerations in mind, it is not a style for everyone. If you would like the modesty of covered shoulders, or a little more comfort while wearing your gown or even a style that defies the norm of the industry, consider these alternatives.
Spaghetti straps - These can easily be added to a strapless gown by your alteration specialist. Straps can also be a fun way to play with the style of your gown. You can cross them in the back, add beads or lace, even make them detachable.
Tank - This term refers to straps that are built in to the design of the dress. It is a very comfortable option.