This is off topic and opinion based, but hey, I’ve been in charge of cms procurement and my day job concerns explaining tech to non-techies, so I’ll give it a go. Most lists of arguments (example) concentrate on price and ease of use for administrators, but that may not be convincing to execs who favour (imaginary) quality over everything.
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Lots of big brands use WordPress. The Walt Disney Company, for instance, is at the moment switching all its sites to WordPress. A quarter of the web is now powered by WP. Even if it were not a good cms, it is becoming the de facto standard. Not wanting to use WordPress is like not wanting to use MS Word.
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If you don’t adhere to standard software, you face continuity problems. What if your vendor goes bust or discontinues the product for lack of customers? You might get seriously stuck. A minimum requirement when purchasing a cms is that it comes with an option to convert all data to an external format. Never become hostage of your vendor. That will never happen to you with WordPress, provided you own themes/plugins you let made to order. Even if Automattic would go under, the installed base is so large another company would take over. It’s open source, after all. And if WP would cease to exist, any new kid on the block would have an importer for WP content.
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Those stories about the limits of WP are something of the past. It’s like complaining about features Windows XP didn’t have. Not relevant anymore. Don’t believe it? Let’s build a hotel site (or any other site with complex functionality). Look who has it ready within a week. For $100.
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WordPress is not something built by geeks in their spare time. Its brand owner, Automattic, is a business that is valued at over 1 billion dollar. Its business model is non-standard, but sound.
That’s my quick thoughts. Feel free to add or edit.