Because your metabolism is slow right now.. Be patient, weight does not magically disappear focus more on having a balanced diet(remember the food pyramid?) and do exercises a few times a week.. Good luck:)
Don't worry too much, you'll probably have a spurt pretty soon and lose some weight and gain some height!
I'm sure you've probably heard this too but eat well, snack on fruit/ricecakes/veg/cereal if you're hungry between meals, watch your portion sizes, and avoid high sugar foods (and drinks)
DONT try starving yourself/cutting food right down, as this will actually low down your metabolism, and make you put on more weight on the long run!
best of luck xx