They didn't support multiple platforms for the same reason Blackberry's BBM was a Blackberry exclusive; platform lock-in. I have iMessage and I love using it with people I text regularly. It still works for SMS, but it's a degraded experience. There are tons of other options now, but I still use iMessage the most since it's the best-integrated messenger on iOS, and they'll keep it that way with things like private APIs + total control over the App Store.