It is probably multiple factors, such as the fact that OnStar requires factory installed equipment. It can't be added as an accessory to the vehicle. That being the case companies other than GM may not want to pay GM the cash for every car produced if it is only going to be used in 10 to 20% of them (and this figure is probably really high). GM is the owner of OnStar, they have allowed other companies to buy the service from them, but still it is like advertising for the enemy.
Also, as was mentioned, so many people have cell phones now that the only time OnStar becomes truly useful is in the event that you can not make an emergency call for yourself and no one else is around. Add to all this the expense that a customer will see for this service ($16.95 a month) which is really high for a service you won't use much given it doesn't give you directions which is probably about the most useful feature for a lot of people. So $199 a year for features you probably get with AAA with a low cost from your cell phone, but for the air bag deployment.
I guess there is the vehicle recovery feature but I would guess OnStar is a pretty big flag to thieves where Lojack at least doesn't have a big flag that says "hey Lojack on board!". I guess you can say there are unique features, but are the unique features worth the added price to every vehicle plus the service fee?