They weren't as open as you remember for most people using pre-iPhone smart phones. Carriers did all sorts of things to cripple reduce functionality so they could sell it back to you.

A major factor of the iPhone's early US success was forcing a major US carrier (AT&T) to stop doing consumer hostile things and just let the device connect to the network as intended.

So, in part, it was improved openness that drove iPhone/Android's success. It just was network openness instead of device openness.