On the back of Poi's recently failed Kickstarter ($27,236 pledged of $80,000 goal), a game which I thought looked really impressive for a Kickstarter and only needed a modest goal to reach success, I looked into the back catalogue of 3D platformers funded on Kickstarter and there seemed to be no consistency to it:

Successes

Yooka-Laylee (PC/PS4/XB1/WiiU) - £2,090,104 pledged of £175,000 goal (May 2015)

The big success story.  Made by former Rare devs, effectivley copying their success with Banjo Kazooie.  Shattered their Kickstarter goal and then some.  Colourful; exploration and collection-based with anthropomorphic characters.  Plays up the N64 connection significantly.  

A Hat in Time (PC/Wii U) - $296,360 pledged of $30,000 goal (May 2013)

Nearly two full years before Yooka-Laylee and when Kickstarter was a much newer idea, a Hat in Time smashed its own funding goal (though by not quite such a drastic percentage).  The game mixes the cel-shaded look of Wind Waker and specifically mentions being influenced by Rare in the campaign pledge.  Human characters, not animals.

Hover: Revolt of Gamers (PC/PS4/XB1 & Wii U Stretch Goal reached) - $116,398 pledged of $38,000 goal (April 2014)

A slightly different game for a change, this one instead being inspired by Jet Set Radio and being a first-person platformer.  Again, pretty blatantly plugged their inspiration and as such easily reached their funding target.

Lobodestroyo (PC/Wii U & PS4/XB1 stretch goals reached) - $43,831 pledged of $35,000 goal (November 2013)

Now, this one is the first one that's looked pretty poor to me.  Although it's a pitch video, characters move slowly, platforming looks boring etc.  But it's another earlier Kickstarter and - like Yooka-Laylee & Hat in Time before it - heavily plugs the N64 connection, going as far as to mention Rareware in the second sentence of the pitch.  Anthropomorphic characters again.

Spooky Poo's Happy Hell (PC - WiiU/PS4 stretch goals failed) - $14,161 pledged of $13,666 goal (February 2015)

The first Kickstarter to not excessively quote that it was inspired by classic games, managed to reach its funding goal but only barely - and was a long way off reaching any console port stretch goals.  Like Lobodestroyo, looks clunky and quite poor.

Failures

FreezeMe (PC/WIiU) - $6,648 pledged of $15,000 goal (May 2015)

The first one not to receive funding is a recent game from this year.  And it seemed to do everything right.  Played up the N64 connection in the campaign (although didn't mention Rare specifically), made sure the only console release was on WiiU etc.  Even stuck in some Mario Galaxy-style levels and an interesting camera-freeze mechanic, yet couldn't even reach a miniscule goal.

Poi (PC/WiiU & PS4 stretch goal) - $27,236 pledged of $80,000 goal (August 2015)

The second failure on this list, also from this year.  Mentioned Mario Games and Banjo-Kazooie in its campaign pitch.  Human characters and seemed to be more Mario-64 inspired.  Lots of media to show off in the campaign yet ultimately didn't even get close to its funding goal.

To me, it seems like the bubble has been bursting this year. Back in 2013 when we saw A Hat in Time (which does look legitimately good at this point) and Lobodestroyo, merely mentioning the N64 and making sure you released on WiiU seemed to be enough to get you funding. This year, two very decent looking games went un-funded despite ticking all the right boxes that previous 3D platformer Kickstarters have done.

It may well be that Yooka-Laylee sucked a lot of the funding out of projects like these. Or maybe it's just been a year of too many high-profile Kickstarters (Bloodstained & Shenmue 3 as well).

One final thing I'd note is - all of these devs appear to think they're going to have their biggest console successes on WiiU. Maybe that's correct, but it didn't save Poi & FreezeMe. I wonder if there's any research out there that shows where 3D platform fans moved to after N64. Personally, I grew up on 3D platformers on PlayStation - so playing up the Rare connection has no emotional attachment for me, but it's a genre I'm still very interested in. No developers seem interested in appealing to gamers like me though, which is a massive shame.

And I'm not saying being on PS4 is going to be an automatic success. GRIP (Rollcage spiritual successor) recently failed a campaign on PC/PS4, presumably thinking that those PS1 gamers moved on to PS4.

Those two criteria apply to 1 of 5 successes.  The "play up nostalgia" applies to 4 of 5 successes and 2 of 2 failures.  Seems inconsistent to me.

I think your second paragraph has better reasoning.

Competition and raised standards have made it a lot harder to succeed. Only the most insane, die-hard fans will support new 3D platformers for the sake of it; the rest will pick the best looking ones and avoid the others.
Looking through all of these games I'm far from surprised that FreezeMe and Poi failed, and that Spooky Poo barely made it. They're simply not good enough, especially when compared to A Hat in Time and Yooka-Leylee. If Lobodestroyo had been announced this year I'm sure it would have failed as well, or at best reached the goal at the last second.