Android Lollipop distribution has been growing quite steadily, but it still lags behind its predecessors. Many devices are being refused the update, whereas other OEMs are just slow to update. The latest report coming from Android shows Android Lollipop breaking 12.4%.
Android Lollipop is still far behind KitKat (39.2%) and Jelly Bean (37.4%), the two most recent OS versions behind 5.0. The number is expected to continue to rise, as Lollipop continues to roll-out devices more and more, but it will take time before it overtakes KitKat and Jelly Bean.
In the meantime, Android M will be released (August) much sooner than Lollipop will hit a good amount of devices, so the chart will continue to skew. Android M will hit Nexus devices as soon as August and other major devices after 90 days.
Note: This data is gathered from the new Google Play Store app, which supports Android 2.2 and above, so devices running older versions are not included. However, in August, 2013, versions older than Android 2.2 accounted for about 1% of devices that checked in to Google servers (not those that actually visited Google Play Store).