There seems to be tons of mods for BGII, but relatively few for Icewind Dale ... just wondering if anybody could shed some light on why that is.

There are a few mods out their, Co8 for example. But these games are very different. Remember, BG I&II, you can only make one player character, so the NPCs are very important. Most mods I seen or played change the NPCs to better powergaming options or change annoying habits of the NPCs. IWD series are mostly linear, yeah you don't have to do all the side quests, but you do have to finish one chapter before going to the next.
If you really want to mod your game, DaleKeeper can change anything about your character, equipment, skill, feats, or spells to whatever you want.
Both these games have so much replayability. you don't really need to mod them to get hrs of fun out of them.

I reckon it is because before people got a chance to mod IWDII, NWN came along. So all the people who want to mod a 3e game are doing it in NWN, given that that appears to be the platform of the future, for better or worse.

Co8 makes ToEE, not IWD2 mods.

My bad on the Co8, I've used them too. Is it the DaleKeeper that mod the opening to undead instead of goblins?

No. Weimer has an undead Targos mod (I don't remember if it's part of another or stand alone). There should be a link or mirror on this site.

Both IWDs have seen less mods than the BG games have so far and that isn't really going to change. Reasons may vary... Well, you don't have a charname with a world-saving history. The game is a bit different from the BG counterpart. And all... But yeah, part of the answer is "I don't know".

Part of the reason might be that people don't have any ideas for making mods for IWD series.

More than half the mods for BG series are NPC mods, which do not have a place in IWDs.
Then there are additional item & weapon mods. I don't think anyone will really miss these, still there are a couple of this type.
Next there are additional kit mods. In BG II every class had about 3 kits, when they could at maximum have 9. IWD did not the capability for adding kits but IWD II has seen a couple.
New quest mods could easily be added into the BG world since there were large number of areas with no quest of their own. In IWDs practically every area is used for the main one-track-railroad quest.
Finally, quite a number of mods for BGs were fixpacks & tweaks. I guess the programmers did a decent job with the IWDs as compared to BGs. Now if only there were a fixpack for IWD Dragon's Eye ...