I wouldn't be at all surprised if the Bean Counters have got a hand in this.
Blues are probably the cheapest form of uniform currently on issue, certainly cheaper than a flying suit/ flying boots etc...
Somebody has probably worked out that it costs less to replace a pair of No2 trousers and shirt than a flying suit or CS95 and sold it that way. Especially if the Commissioned Types have to buy them.
Likewise, there's probably a limited budget for CS95, stocks of which are probably controlled by the Army.
I remember back in the 80's/90's that flying boots were considered 'gucci' for wear on guard/exercise (pre boots combat high) and getting your mitts on a pair was a major coup. Some bright spark worked out that the RAF's yearly total of flying boots issued added up to each aircrew member wearing out 7 pairs of boots per year!