There are several reasons,
- Fixed supports prevent both horizontal and verticle displacements. This will cause the development of thermal stress during temperature fluctuation.
- Fixed supports are very difficult to implement(and maintain) from a practical point of view. This can cause disaster if a fixed supported end becomes simply supported over time due to some joint failure.
Consider one deck of a bridge supported over two piers(see the image). There’s a concentrated load at the midsection of the span. If it is assumed that the end of the span is ‘fixed supported’ the maximum bending moment will be much lesser than if it were assumed to be simply supported. There’s a vertical shift in the BMD between the ‘fixed supported’ and ‘simply supported’ beam. The maximum bending moment determines the most critical section of the beam. The dimension of the crosssection as well as the reinforcement(if the deck is made from concrete) is provided based on the magnitude of the bending moment. Consider a situation where a beam was designed with ‘fixed support’ and for some reason (damage due to natural wear and tear) it gradually stared to loose the ‘fixed-ness’ of the ‘fixed support’ and ultimately became a simply supported beam. There will be a large increase of the bending moment in the midsection of the span. The beam was not designed for that large bending moment and might cause failure. So, considering a bridge with simply supported edge adds a safety-net in the design process.