how to transfer whatsapp data from android device to iphone

According to WhatsApp themselves:

https://faq.whatsapp.com/general/account-and-profile/about-changing-phones/?lang=en

If you’re moving to a different type of phone:

• You’ll keep your account information including your profile photo, name, about, individual chats, group chats, and settings
• You can’t transfer your chat history

Note that there are some services out there that purport to move your chat history. They do it basically by downloading all your history, thereby giving them the keys to your account (username, login and encryption keys). I wouldn't recommend them.

nj180 wrote:

why i can't transfer my whatsapp data feom android phone

You would need to ask Whatsapp why, as it isn't an Apple or iphone issue.