Because it's the only savings vehicle that's tax advantaged on the front-end and the back-end.
You're saving and growing (brokerage option) money for health related expenses over time instead of back-loading them.
I'm in my 20's and on the HDHP with an HSA. The way I see it: (a) max out of pocket is similar so the same risk to me exists, (b) the monthly HDHP premium + the HSA contribution is similar to the monthly PPO premium so the cost is similar, (c) if the costs and risks are similar, I prefer the growth, tax deduction, and savings offset for later in life instead of the sunk cost of the PPO premium.
After age 65, you can withdraw funds for any reason without the 20% penalty (if not used for medical related expenses income taxes will apply) so it in affect becomes a way to boost traditional IRA contributions by 60.00%. I welcome the tax deduction for saving that I intended to do in the first place.
If you do use it for medical related expenses, then income taxes do not apply and the government funded 25% or 28% or 33% (or whatever tax bracket you're in) of that expense.
This post was edited on 1/13 at 1:35 pm