Several years ago, Sony did have PS Now on a wider range of devices, including PS3, PS4, PS Vita, PSTV, and some smart TVs and smart Blu-ray players from Sony and Samsung.
Around mid 2017, they added the ability to stream PS4 games, and also brought streaming to PC. Around that time, they removed they ability to stream PS Now from everything other than PS4 and PC.
This wasn't done because Sony wanted to "restrict access" to the service. Rather, it's because many of those devices were older and/or have limited processing power. Either the devices were not able to handle the updated codecs to decompress and display the video at an acceptable level of quality, or (if they could) their usage metrics showed it wasn't worth the development effort for niche devices with limited use..
You'll also notice that many other video streaming apps were also deprecated on those same devices around the same time. It's not a "PS Now" issue, it's an "aging hardware" issue.
Sony has repeatedly expressed their intention to eventually expand their range of devices that can access PS Now again. We can only hope that they do this soon.