Same reason the Corsair fighter has bent wings. Makes clearances easier.
Look at the lumpy, sticky-outy things on a tank car. You have the trucks/bogeys at the end bottom... and the massive fill valve and work safety area at center top. Lowering the center is advantageous to the car's clearance envelope, allowing it to have more tank volume in the same clearance plate (loading gauge for Brits, not that it would fit in any British tunnel save for the Eurostar).
For the last 100 years, railroads have been careening toward heavier and heavier cars, while many older system tunnels do not have high clearances. (Lines commercially viable for double-stack container service are being notched for double-stacks).
Assures head space too. While the bend eases unloading, as NMech covers amply, it also prohibits entirely filling; there is an airspace above the load that simply cannot be filled, short of drawing a vacuum on the tank (which the Mythbusters showed is a bad idea).
Loads have different volumes at different temperatures. Notice the steam-heating apparatus in the cutaway on the drawing. That means the car is intended for loads that may be too thick to flow without heating. That material's volume will increase while you are heating it, or simply the inevitable effect of 16 hours in the Arizona sun on a car painted flat-black. You do not want the car to "hydraulic lock"; that would send pressure up very rapidly and crack the tank.
That "unfillable" volume assures there is head space for the liquid to expand into; of course this compresses the gas there somewhat, increasing pressure on the whole vessel; but nothing it can't handle.