That is because sending Loras would earn him friendship of Highgarden with her colossal resources and armies. And a man who had Lannisters for enemies would do well make friends with Tyrells.
If Loras succeeded in his mission, that would make him feel grateful towards Eddard for giving him a chance for vengeance. Eddard could further cement that alliance by betrothing Arya or Sansa (Once she was free of Joff) to Loras.
If on the other hand, Loras died on hands of Lannister bannermen, that would bring Tyrells to Eddard's Camp so that they could take their revenge. And in case of Petyr Baelish, it helps prolong the war he was brewing, weakening the great houses.
It was a win-win scenario for Eddard Stark and he blew it.
Varys saw things the same way and he and Eddard had this conversation:
You are a bolder man than I, my lord,” the eunuch said softly.
“How so, Lord Varys?” Ned asked brusquely. His leg was throbbing, and
he was in no mood for word games.
“Had it been me up there, I should have sent Ser Loras. He so wanted
to go... and a man who has the Lannisters for his enemies would do
well to make the Tyrells his friends.”
AGOT: Chapter 43