They don't last forever. If you dead head the flowers, the plant will make more.

Also check to see if you have bugs in them. Also have you had a lot of rain or been watering the flower area? This will tend to make the flowers go brown as well.

Test for thrips by tearing open spent bloom. See if they are not host to little brown insect. Sevin is a good control. Steve in Baltimore County.

I once read that it is worthwile spraying with a fungicide, it hold off the inevitable "dieback" of the flowers, especially with rain and humidity.
Some times I will just pick off the browning pettals on the large dahlias that continue to grow.
another way to look at it is that, if you cut the blossom off it will encourage more!
good gardening.