Actually, there is. The sleep widget is a function of the on-device sleep tracking licensed from First Beat. Garmin did not license it for older devices like the Instinct. As the Instinct Solar is just a fancy, facefilftes version of the Instinct (from the point of view of license givers), the sleep widget is unlikely to be backported to the Instinct/Instinct Solar series.
On the other hand, that allowes sometimes allows Garmin to backport new licenses to older devices while developing those facelifts. They gave the original Fenix 5 the Stress Tracking after buying it for the Fenix 5 Plus, and brought the Body Battery back to the original Instinct as they bought the license for the whole Instinct line up while developing the Instinct Solar.
The sleep widget is a function of the on-device sleep tracking licensed from First Beat.
Garmin actually acquired the Firstbeat Technologies corporation a few months ago, so the licensing should not be a concern any more (unless they did not purchase the complete control over the license rights too, which is very unlikely).
The reason why Garmin does not and will not add a full bunch of functionality available in other watches may be more prosaic - in comparison with the Fenix, Instinct is a low cost model. It has also a much smaller memory. All the additional features may simply not fit into the firmware. Each additional feature also increases the support and documentation costs.
But even if Garmin could perhaps implement all the features of Fenix into Instinct, there would be then no reason not selling it for the same (or higher) price as Fenix. And that's not what anyone would like.
