I guess if you increase the number of polar groups which aid removal and metabolism (reducing half-life) you make it far less likely to enter the brain and be a useful sleep aid.
Possibly why there are no short acting anti-histamine sleep aids? (although someone might know of one I don't and I know hepatic metabolism is far more complicated than just polar and non polar elements)
Also anti-histamines tend to have many other low potency pharmacological effects (e.g promethazine is something like a 1/10 th potency of chlorpromazine as anti-psychotic) which is fine left alone but I guess if you start changing functional groups even in small ways the result could quickly be a entirely different compound.
I think the first SSRI was derived from an anti-histamine...
http://en.wikipedia.org/wiki/Zimelidine