why should I backup zcash wallet regularly On zcash official website, making regular backups of the wallet is recommended.

When I use bitcoin, I have a master private key, and all private keys can be generated using the master key. So I can only backup my wallet once.

So what's different in zcash? Why should I backup the wallet regularly?
The BIP32 specification allows a user to derive many keys from a master extended private key, and this master key is derived from a seed (often, a mnemonic phrase). This allows the user to just back up one piece of data (the seed phrase), knowing that they'll be able to use it to derive all subsequent keys in their wallet.

Prior to the implementation of BIP 32, most wallets were a collection of keypairs that were independently generated at random, so one key would bear no relation to the next. In this style of wallet, every time you generate a new address, you need to save a new backup copy that includes the new address. Older backups would not contain the info that allows you to derive the same new address.

I am not extremely familiar with Zcash, but I would guess that not all implementations of their wallet software use a BIP32 style wallet architecture, thus the recommendation for regular backups (similarly, not all bitcoin wallets use BIP32).