Mummies of animals, even frozen as the mammoths were is rare, exceedingly rare. And the salamander wasn't frozen. So different preservation circumstance, different outcomes?
The same sort of question could be asked about why if tissue millions of years old can be preserved and found why would tissues just thousands not be more common than it is.
And yes the assumption was made that the fossil was millions of years old:
“(PhysOrg.com) — Scientists have extracted organically preserved muscle tissue from an 18 million years old salamander fossil. The discovery by researchers from University College Dublin, the UK and Spain, reported in the Proceedings of the Royal Society B shows that soft tissue can be preserved under a broader set of fossil conditions than previously known.”
“...soft tissue can be preserved under a broader set of fossil conditions than previously known.”
OR perhaps the fossils are not as old as thought.
From the introduction:
Herein, we describe, to our knowledge, the first record of organically preserved musculature including its sedimentological context. The muscle's gross morphology resembles that of an extant analogue, but this, alone, is not the basis for our conclusion. Remarkably, despite some degradation before fossilization, diagnostic macromolecular ultrastructural features have been retained.
From the materials and methods:
(a) Fossilized muscle tissue
Samples of muscle tissue identified under a binocular microscope were picked from the specimen using sterile scalpels and needles. For scanning electron microscopy (SEM), samples were not prepared further; they were mounted onto aluminium stubs...
And the conclusions:
4. Wider implications
The detail revealed by TEM imaging unequivocally identifies the organic remains as fossilized musculature from the salamander itself. This therefore confirms, for the first time, to our knowledge, that the high-fidelity fossilization of extremely decay-prone tissues as organic remains is not only feasible but can occur in the absence of protective encapsulating agents such as bone (in the case of the bone marrow, McNamara et al. 2006) and amber.
Nowhere in the paper are the terms "Fresh meat" used. They state throughout that it is fossilized muscle. Therefore BTMS* Lied.