UTC was proposing the "slimline" MS-110 pod back in 2015.
Litening 5 is no rubbish, however. Among other things it is intended to help targeting for Spice guided bombs (which use EO scene-matching guidance) beyond the range at which l@aser designation works, so it has bigger optics than the earlier versions as well as being fused/multispectral. So it may have been decided that the difference between L5 and MS-110 is not worth the cost of buying and supporting a whole new pod.
The current EOTS on the F-35 is not in the same class; the Advanced EOTS is not under contract as far as I know, and even if you have a fabulous sensor it helps to have a way to record its data and get it off the airplane.