The reason is that everybody knows they can sell Bers. So they buy them in bulk from websites.
Which leaves the supply of Jah runes lower than that of Ber runes on jsp. For runewords that require both, like Enigma, they need an equal supply and when the supply for either is lopsided, the demand for both will cause the price for the lower in supply to increase.
The price of Jah runes then increased as an incentive. It's economics.
Also, people still roll for perfect gear later in the season.