Bridge & other card games/Jacoby transfer
what are the reasons behind "Jacoby transfer is off" after an opponent overcalls. why I cannot bid 2D to show my 5 or more cards hearts if there is an overcall of 2 Clubs.
It depends what suit the opponent overcalls. If the overcall is 2D then it makes more sense to be able to bid both majors naturally rather than using 2H as a transfer and not being able to show a heart suit at the two level. Similarly, if the overcall is 2H, you need 2S to be natural.
However, if the overcall is 2C, many people continue to play that transfers still apply. A double the 2C bid would be Stayman and 2D and 2H would continue to be transfers. Therefore you can have your wish.