I’m not certain why they have HSA after 401k and IRA, but some possible things I can think of:

• HSAs can be harder to take advantage of of the triple tax benefit if you’re retiring early (that is, still younger and healthier)
• HSAs probably have worse investment options than an IRA
• Allowing the user to optimize their Roth vs Traditional mix

Again, I don’t really know because you’re right about the HSA triple tax advantage making it seem better than IRA or 401k, but I’m sure there was a reason given if you care to trawl the subreddit.