I'd have to agree that there seems to be fewer mods for the newer versions and those that exist tend to be smaller than previous ones. I suppose this has to do with changes Mojang made to the code after 1.7.10 which (as I understand it) made modding much more difficult by removing certain graphical features and requiring modded items to have separate files among other changes. Combine that with lower fps due to the new, beefier Minecraft code, the fact the code changes more frequently now (requiring complete mod rewrites), and signs of the games popularity beginning to fade and it's no wonder modders are less active. Mod authors just don't see it worth there time anymore.
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1.7.10 lasted for way more than 2 years, whereas 1.10+ only have a few months in between. The issue is, 1.8. and 1.9 were a non-factor in mod development. There was just way more time to develop for 1.7.10. In addiiton a lot of people dislike certain changes done since 1.8 like the new block rendering system, the move to more data-driven development in form of json files and the combat update. Most modders moved onwards to 1.11 and 1.12 really quickly due to how easy the update-process was (as long as your mod didnt have a ton of legacy code like EnderIO or Buildcraft)
A lot of people think 1.12.1 will be the next 1.7.10 in regards of its lifecycle. I think the update to 1.13 will be hard and we will lose a fair amount of mods (again), but i dont think it will last as long as 1.7.10 did.
The main factors are: 1.7.10 lasted a long time, 1.8 added rendering changes which makes updating hard, and the actual versions are too "young".
People are mainly waiting for another version stall to start modding heavily. Some people are lazy, others are too busy.. As usual with Minecraft mods, chances are that someone will make their own version of the not-updated mod.
Updating a MOD from 1.13 onwards should be simple but the update to 1.13 will probably really harsh. A lot of mods use the metadata system for their blocks (and sub blocks) which is being removed in 1.13 and we don't know how much work it will be to move to the new system. Also, dinnerbone said they will update lwjgl which can have a major impact on mods that do fancy rendering. But we will see. All I know are tweets from dinnertime that say it won't be easy to update :/