just unlucky I guess, though I have to admit, thinking about it, I generally find more spawners without chests than with. This is something I regards as good luck though, as there is no disappointment when you open the chest with great expectations, only to find a manky loaf of bread.
Well I must have been really lucky for a long time. The only spawners that I have come across in the past with no chest were cave spiders and silver fish.
seed is -6120648606982687686
zombie spawners I found are at 6 23 224 and -203 60 118
Stay fluffy~