I would say it is more a question of the modders having done such a great job that there are fewer bits of the UI left to mod. After all there are only so many ways that you can mod the hotbars
Initialy there are no mods and people go crazy writing mods but as the game matures the rate of modding has to slow down as all the pieces are modded with all the options people want.
There are over 800 mods listed on this site, I guess people just got what they want
Having said that you do see updates fairly regularly to the popular mods (27 updated mods in May is about 1 per day!), so I would say the slow down is perfectly normal and not a reflection of a reducing fan base.