* "Time to ship"
Going the "native" way for desktops means doing specific work for Windows X, Windows Y, Windows Z, MacOs Whatever, than MacOs Whatever-Beta-Plus.
Even using the "write one, use everywhere" framework entices such work, because of the dirty secret no one talks about : they don't really work besides simple apps.
The choice of leaving those problems to the Chrome team is _really_ tempting.
* Availability
Popular Desktop OS have not historically integrated "app stores" or "canonical way to install an app".
Asking them to "Install" stuff is great if they're nostalgic geeks like us ; but real people learned long ago that anything asking the right to "install" on their computer is malware from a shaddy gambling website.
* Discoverability
People don't care about your app. Really, they don't.
But they have chrome on their computer (it came preinstalled, and they need it for Facebook), so at least you get a chance to grow your user base if you manage to put a link to your website in your facebook feed, or, if they're high-tech users, as an ad on the top of their google search result (also knows as "the internet".)
The only exception, as usual, is games, because: * they need to run native code and can't afford the dozen layers of inderection of a browser * their users are dedicated nerds who will happily glue toxic chemical products to the CPU of their opened motherboards in order to get 1 more frame per seconds * the install process of game exists, it's called Steam