A number of reasons.
I don't fully understand how they are priced[1] and it's not immediately obvious whether you will get your capital back or not at maturity. Typically I would expect you definitely would but, for example, it's not obvious to me that
AIUI, the OP is looking for a low to no risk way of getting capital gains (presumably to use up his CGT allowance). To get that low risk he has to hold the gilts to maturity
AFAICT All of the short term (