Why are some words in menus italicised?

Loanwords, i.e. foreign words used in Estonian, are written as they are in the foreign language, for example buffet, crème brûlée, cappuccino. Loanwords are declined using an apostrophe, whereby the case ending is not italicised, e.g. latte macchiato’t, teriyaki’ga, brownie’sid. If the loanword is part of a conjunction, it is separated by a hyphen, e.g. cappuccino-kook, tomati-mozzarella salat, wasabi-kaste.

Why can I not use the word passion?

Because in English it means something else (https://sonaveeb.ee/search/unif/dlall/dsall/passion/1) from the term you want to use in your menu (the fruit is called passion fruit).