At this time SMS is the only texting service that can be used for all phones. All others are proprietary, meaning you have to install software and use and use that companies servers. iMessages is no exception.
That is why you need an iPhone. You need a phone with SMS, and Apple only supports it's own phones. To be fair, supporting every kind of phone out there would be a major headache.
The alternative would be for Apple to give you a phone number, which would make them a phone service, or at least closely partnered with one or more phone services. I doubt they want to deal with that.