Subs are weight limited. There is current trend to make control surfaces out of glass fiber to save weight. The vessel has to be buoyant despite 80mm thick steel skin. Spain just had trouble with too heavy sub. http://www.dailymail.co.uk/news/article-2336953/Spains-1-75b...

Long cables are strength limited. At some point you have to put floats here and there so it doesn't break under it's own weight

And then there is the parasitic drag. Long cables have large surface area.