They promised the world, their management didn't understand what OSS meant. It was pretty bad early on.
Their investors also didn't understand OSS and thus made sure that core parts never became OSS.
That's why there was NemoMobile, it was (is?) a true and fully open source implementation, replacing all the proprietary Sailfish bits.
It could have been great, but many mistakes where made in many places, not just by management. Now it's a fringe OS, with a closed source UI and no noticeable market share. Let alone growth in such.
NB: This is based on my personal involvement up to 2015-ish.