They answered that "it's hard to implement modding system so no mods" or something like that.
I will never understand that retarded argument.
All you have to do to "implement" a "modding system" is to have an open enough file structure that people can edit things. Paradox games and Total War games have no mod tools but there are active modding communities because the file structure is sufficiently open that you can change statistics and some mechanics in the game by editing text files, add new units, change the properties of playable nations, etc. Yeah they're strategy games but in an RPG this could at least allow you to modify equipment stats, add new equipment items, etc. It's not much, but it's something, and there's literally no effort involved on the side of the developer. You don't even have to publish a modding tutorial for your community, they'll figure it all out by themselves.
Also, don't they have tools with which they're building the game? Bethesda just released the same editor they used to build Morrowind, Oblivion, Fallout 3, Skyrim levels to the community so they can build levels too. Simple.
Thief modding started when the community asked the devs "yo, wanna give us the editor for the game so we can make our own levels?" and the devs said "k" and they didn't even give them a big tutorial. The community figured out the intricacies of the editor on their own, and by now can even do things the original devs would never have thought possible.
The only possible argument you could make in favor of "it's hard to implement a modding system" is that if you use a third party engine and third party tools, you're not allowed to just release the tools you use to the community, but that's just a legal hurdle, not a technical one.
Allowing mods isn't hard. You don't actually have to actively do anything as a developer to enable modding.