It's about the frame rate and the sensitivity (ISO) of the camera.
Slow motion video is recorded at 120 pictures per second, so there is less than 1/120 second to expose each frame in the video, where a normal picture takes maybe 1/20 second in low light = maximum 20 frames per second.
If there is not enough light the camera try do its best.
60fps isn't slow motion but if you record a slow motion video and play it back in something like MPC-HC you can look at the file properties and it is 120fps :)
That's because 1 megapixels, well 921,600 pixels to be exact is 720p which is the resolution of slow motion videos on the Nokia 8.
Even 1080p is only 2.1 megapixels so your wish for 5MP at 120fps is not going to happen, the Snadragon 835 simply doesn't have the grunt.
I don't think there's a single smartphone currently on the market capturing slow-motion video with 5 megapixel frame resolution. Looking at the phones on the high end of camera spectrum, Galaxy S9 supports at best 1920x1080 at fps (so 2 megapixel) of continuous video (I'm ignoring the gimmicky 0,2s 960fps mode with lower resolution) and iPhone X and 8 seem to have the same constraints.
As for increasing the resolution, assuming what's been said on several web sites is correct and the sensor Nokia 8 uses is IMX258, then 1080x720 at 120 fps is the best the hardware supports as far as slow motion video is concerned.
I did leave a better explanation but sadly the post never showed up as it seems random posts require moderation so will no doubt appear much, much later and seem very much out of context, sorry about that.