I still don't quite understand why pros prefer tubular over clinchers. The only real reason I've heard is that in the event of a flat, tubular tires don't let out air as fast so are safer. I don't know why that would be the case.
As far as the air escaping faster thing goes, the theory is that it's because air escaping a clincher tube can leave freely via the spoke holes in the rim, giving it more places to go. On a tubular, the tire casing totally envelopes the inner tube including from below, so the only spot where air is escaping very rapidly is the hole in the tire.
Whether this is actually relevant or not... who knows, I've never seen it quantified.
The more common safety argument is that a tubular tire stays mounted better when flat. Clinchers are primarily held in place by air pressure, whereas tubulars have air pressure and glue sharing the effort. The fear is that clinchers are more likely to roll, leaving you skidding out on bare rims.
The real reason that I think tubulars are preferred for racing is related to the above paragraph, but slightly different: it's somewhat practical to keep riding on a tubular even when it's totally flat. It's not comfortable, it's not good for your equipment, you need to ride gingerly on it, but it allows you to make forward progress until you can get support.
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Plain old performance is another reason that's often given. In particular, they do save some rotating weight, and people who are constantly accelerating obsess over that. Based on the napkin math I'm not entirely convinced the gains work out how people say they do, but it's a major argument that gets used. And admittedly, crits are
vastly more erratic than most of the riding I do.
And one thing that probably used to be fairly significant: tire availability. Currently there's a nice selection of good clinchers in many shapes and sizes, but that hasn't always been the case.
Heck, even today, you can't get silk clinchers. There are only a few makers of such tires, and AFAIK they're all tubular-only.
