"Because we, the electorate, put her there. Not directly perhaps, but anyone who is prime minister is 1) a directly elected MP, 2) appointed as PM by other directly elected MPs responding to the political situation created by how the electorate has voted. It rarely reflects the opinions of everybody, but tough luck that's democracy."
It doesn't appear to be a commonly known fact, but the Tories won a majority in Britain. Since the British parties don't stand in Northern Irish seats, they stand no chance there, and it's only in the whole UK that they failed to get a majority, but this is really a type of super-majority, because of those extra seats. The great irony of having to rely on DUP support is that the only reason they have to is because of Northern Ireland in the first place.