Why aren’t hydrogen fuel cell cars currently available to the public?

The simple, immediate answer is:  Because the public hasn't shown
any interest in it yet. 

That's why nobody is manufacturing and selling them yet ... there is
no line out the door and around the block of buyers with fists full of cash
looking for hydrogen fuel-cell cars.

To find what you're actually asking for, we have to look one layer deeper.
WHY is there no market for such a car yet ?

-- If you want it, then they build it.  But they will be built in small numbers,
so they'll be expensive for quite a while.

-- The fuel distribution infrastructure doesn't exist.  In high school English,
that means that if you drove one of these, you'd always need to plan your
moves carefully and in advance, because there are hardly ANY hydrogen
filling stations yet.  There's NO place in the USA yet where you'll see four
hydrogen stations on the same corner, all competing for your business and
all making a decent living.

-- There's also no fuel manufacturing infrastructure ... no hydrogen
refineries, where the stuff would come from IF a thousand filling stations
sprang up tomorrow.  So the stuff will become available little-by-little, and
when it begins to appear, you'll need to be really really committed to the
idea of the fuel-cell car that you just bought, because the price of the
fuel will be astronomical for a long time.

-- The existing big "energy" companies ... the ones with names that
everybody recognizes right away, that rhyme with DT, Loyal Clutch Smell,
DrexonFrobil, Howdy Salamco, Levron, MonacoPhllipo ... as well as the
several WHOLE COUNTRIES that make their living from the Billion$ and
Ten$ of Billion$ that they've plowed into finding, drilling, pumping, pipelining,
and refining petroleum, and then delivering, pumping and selling gasoline at
the thousands of filling stations ... those guys are not exactly encouraging the
world to go after a new fangled idea that'll eventually put oil out of business.    

When the population of the world basically rises up and demands hydrogen fuel-cell cars, loudly enough so that the big oil guys can't resist it any longer without looking like crooks, THAT's when they'll do the big turnaround, start
insisting that the fuel cell is the best thing for us since fruit, and that it was
really their idea all the time.