I believe that insurance companies see older vehicles as more of a risk because the owner is less likely to care about them than what they would a car that is worth something.
For example, we have a 98 Beemer 318 iS that is realistically worth no more than £1500 (and it's only possibly worth that because it's 1 owner from new with 48K on the clock and FSH). We paid £600 for it last year and have grown somewhat attached to it, primarily because of it's reliability and the fact it's still worth spending at least £500 on it without question if there is a major component failure somewhere.
While I'd be a tad pissed at losing a decent car if it were to get smashed or stolen, we're not gonna shed any tears over it like we would a car that was worth (say) £5K.
That doesn't mean to say that we drive with any less attention to what is going on, nor do we walk away without ensuring it is fully secure when we leave it.....but the insurance company has no way of knowing that so I guess they have to price based on what the statistics tell them is the "likely" scenario.
Obviously not a fair basis at all for the most of us, but I don't see what we can do as they have us by the short & curlies (and they know it).