Age, bad water condition and possibly a bad installation. But ten years is not out of the norm. Mine failed with the same blisters at about six-years-old, which was premature, caused by previous home owner's pool guy's bungling of the pool and its chemistry. Yours sounds to be a similar story. Plaster can last anywhere from a few years to 20 or 30 or more. 10 to 15 is typical. If your pool was not well cared for, 10 was generous. As long as the pool is holding water, the blisters are only cosmetic, and you could live with them indefinitely, if you didn't mind them. Otherwise: full chip-out and a new finish (which could be some other, newer-type finish that will outlast plaster: pebble, quartz, etc., if you want to upgrade).
New plaster entails a pool plaster contractor that will come out and empty the pool (or direct you to do it). They send out a crew on day 1 and they'll chip out all your old plaster. Next day a second crew will come out and "spray" on the new plaster finish and trowel it smooth. Third day they'll come back and finish up the plaster and fill the pool. YMMV, but that's typical (if the edge tile or coping doesn't need any attention).
When I had mine redone, the contractor removed my main drains (at my request) and converted by straight-pipe returns to adjustable ones. You can do that and more. Change the color, of course. Remodel edge tile if you want to get into it. They can even do minor shape changes (adding a bench or sun shelf, reshaping stairs, adding hand rails, etc). Interview some contractors, ask about prices, finish options and all the other options. Just a matter of how much you want to spend.
If this is not your forever home, and you might move in 10 years or less, get plaster. It's the cheapest and is one of the nicer feeling finishes (smoothest). Don't get trapped by thinking a high-end finish will fetch you a better home-sale price, it won't. If this is your forever home, consider a longer lasting finish.
