* not everyone can be an expert on everything;

* people can't know what they're not sufficiently knowledgeable about;

* people would like to vote (if it was quick and easy) for anything they have even the slightest opinion on;

* people could be manipulated much easier than an expert or than an educated representative influenced by experts would;

* people value their voice and opinion and themselves too much;

* only a minority of people would vote on lots of things, skewing the results; a majority would vote on just a few issues;

* education fucking sucks everywhere - people don't have enough information about different topics, they don't know how to get said info, how to analyze it or how to filter trash or spam;

* people passionate enough about something will vote on it much more than people not passionate enough about it - with the caveat that someone can be passionate "for X" but not that passionate "against X" - which can lead to the phrasing of the question deciding who will vote;

* it would be easier to bribe someone to vote on something they don't care about (or don't realize they care about) - you wouldn't vote for a new supreme leader but might vote for a specific change in laws about metallurgical unions (gave it as an example as I know nothing about the topic so I "don't care" about it).

If people were educated, had critical thinking, knew how to spot manipulation, weren't greedy and were able to think about abstract things, direct democracy might work. But people aren't, don't, don't, aren't and aren't.

The idea is basically that you give a politician a mandate to use your vote. Whatever your chosen politician votes for will count as their and your vote. If you happen to disagree with your chosen politician on a given question, you can manually vote in that question. Your chosen representatives vote in that question will then be worth one vote less, since you've effectively used it yourself.

In the end we get the best of both worlds: voters don't have to vote in every single issue, but they can should they choose to. When they don't vote themselves, a politician they've chosen gets to use their vote, in a representative-like manner.