Here’s why iMessage isn’t available for PC yet:
iMessage is developed by Apple for macOS, iPhone, and iPad;
Apple only develops apps for its own devices (Macs, iPhones, and iPads)
You see, Apple wants you to use it’s own devices so it doesn’t release its apps on others platforms.
iMessage was launched on June 6, 2011, by Scott Forstall at WWDC 2011 as part of the iOS 5 software update that became available on October 12, 2011. On July 25, 2012, Apple introduced OS X Mountain Lion. One of the new features was iMessage, the replacement for iChat.
With iMessage, you can send messages, group messages, photos, videos, contacts, locations, and files to other iMessage users. It is thus, if an Internet connection is available, an alternative to SMS and MMS.
iMessage is accessed via the Messages application on an iPhone, iPad or iPod touch with iOS 5 or later and Mac OS X Mountain Lion or later. It uses wifi, 3G or LTE.
Users can register with Apple to one or more email addresses to use iMessage. iPhone users can also use their phone numbers instead of an e-mail address, which with iOS 6 or higher also can be linked to an e-mail address. When iPhone users want to use the app with a data connection, messages will first look whether the recipient also has iMessage. If that is the case, iMessage will automatically send a message. If not, it will send an SMS or MMS instead. A blue send button refers to iMessage, a green send button to SMS or MMS. One can see in Messaging whether a contact is typing a message. In addition, one can see whether a message has been received or read. All iMessages are sent encrypted.