Apple's cut is part of it. However, since overall fraud rates are about 0.05% of purchase amounts, Apple Pay's 0.15% costs more than fraud does. Especially for worldwide banks that have used EMV for years. (Debit card fraud is lowest of all, at only about 0.02%, because of the required PIN even in the US.)
In addition, the issuers are required to set up and maintain servers to allow Apple's server's to request registration credentials, the issuers must create and maintain their Java applet in the iPhone's secure element payment processor, the issuers must add tokenization support or pay an outside firm fees to do so.
Finally, the issuers are also required to create reports to send back to Apple, with dozens of informational categories, such as number and dollar volume of credit and debit activity, average ticket, breakdown of transactions between in-store and in-app usage, and top 100 merchants by charge volume. (This way, Apple can continue to publicly say with a straight face that they don't collect info at time of purchase. What they don't publicly talk about, is that they get a lot of useful aggregate info kicked back to them later on.)
All of those are extra costs and effort.
The main reason banks sign up is because they are scared of their cards not being used, as contactless phone payments slowly begin to take off. (See comment above about wanting to be the default payment card.) Basically they're paying Apple a ransom, since unlike other electronic wallets, Apple demands payment for doing little more than providing the hardware.