Well, for starters, it's because your body may not want you to. ;) 5 lbs is well within the normal variance for weight, and we often choose arbitrary numbers for our goals that may not be realistic for us to achieve. Given that our weight can fluctuate as much as 10 lbs in a single DAY, 5 lbs to go may simple be too tough.
Also, are you actually eating enough? With that little left to lose, you only really can expect to lose around .15-.25 a WEEK. That means your calorie deficit is going to be very small, with little room for error. You're well past the point where 1lb per week is possible.
Instead of focusing on the scale and your calorie burn, why not aim for composition, and start adding strength training if you haven't already? Your weight doesn't tell you how your clothes fit, nor how much energy you have.