First of all, I'm not an expert on the topic, so if somebody finds something that's not correct in my post, please note and I'll fix it. But let's try with the little I know:
Because UK doesn't want the current one, which is "the four freedoms", meaning free movement of goods, capital, labor and services in the single market (EU countries + some special cases, such as Norway which has made a deal to be part of the single market, without being a EU member) area.
I am sure EU would offer this deal for UK, if UK wants to accept it. But the UK apparently doesn't want to have the free movement of people (labor) so it's rejecting the current deal.
Previous answer + if Britain was given a special deal, having access to the single market, without having to accept the free movement of the people, this would mean that EU member countries have a worse deal than Britain has. Free market (like the single market) is about both freedom and a risk. Britain would have the freedom without the risk.
Who wants to be in a union which offers better deals for outsiders than for it's own members? Every EU country would want out of EU after that. It would be a suicide for the union to give Britain "single market a la carte".
Yes there are other reasons. They don't want to destroy the EU by giving Britain a better deal than the members have, it would be a suicide move for the EU. Would you like US to give more benefits, social security and freedoms for all Brazilian citizens than for the US citizens?
I know Norway has made a deal which includes it in the single market, which means it accepts the four freedoms. I'm not sure about Switzerland, but I suspect the position is somewhat similar. They're pretty close to being members in economic sense. I repeat, I am pretty sure UK could have the same or similar deal Norway has, if it just takes it.