Because HSA has triple tax savings.
Typically, a 401k or tIRA has double tax savings, ie, contributions are tax-deferred, and then growth is tax-free. You will be taxed at your current tax rate upon distribution/withdrawal.
For an HSA, contributions are tax-free, growth is tax-free, and distributions are tax-free as well, as long as used for qualified expenses. But after age 65, you can take a withdrawal penalty free (not tax free), thus the HSA functionally acts like a tIRA. After age 65, you can also use your accumulated HSA funds to pay for medicare premiums, tax- and penalty-free.