It wasn't necessary: the sun-defying shielding could be implemented as a mere software command to standard shields.
We have no evidence that this shield would protect against anything else besides stars. It was never said to be particularly effective against phaser fire, for example. So Starfleet wouldn't be especially interested about it, except for those rare cases when there is a need to dive into a sun for a while. No doubt Janeway's ship had a metaphasic setting to the shields as one of the eleven thousand options - she just never had the need to apply that particular setting.
The armor of the Defiant might really be something that never panned out enough to be entered into Starfleet public record. Sure, it worked; but like with so many military inventions in the real world, its working was not relevant enough to cause a general reaction. Stealth through special radar-absorbent paint is old news (at least three decades old, and conceptually probably as old as radar itself), but that paint is not applied on all aircraft because it's only effective in combination with other special measures. Ablative armor might not make a difference unless applied on a very small starship of a specific shape.
Come to think of it, did ablative armor ever make a difference? It helped the Defiant take a few unshielded hits from a Klingon capital ship in "Way of the Warrior", but other starships have survived unshielded hits like that, too. It supposedly mattered enough for Benteen to yell to Leyton about it, but even Benteen never quoted it as impregnable or anything. And during that battle in "Paradise Lost", the Defiant never was said to have lost shields anyway.
So I have no trouble accepting that ablative armor might be a curiosity that only certain field commanders would originally be familiar with, much like certain makeshift up-armoring measures taken in the real world. Benteen and Worf might be fighting in tanks, for example, and Benteen would be annoyed that nobody told him Worf's tank was equipped with anti-RPG wire mesh. It wouldn't really make any difference in the battle as such, but if Benteen had poor intelligence on one area, she would be wise to suspect that her other intelligence was incomplete as well. And that would be serious. What else was Leyton hiding from her?
Timo Saloniemi