I think that it's because 10 is more than 9.
The real question is: "Is it better?" I think that depends on what additional cog the 10-speed happens to give you.
Mountain bike cassettes haven't come out in 10-speed yet so it's not going to give you an easier hill climb gear. That leaves closer spacing as the only possible benefit.
At the high gear end of the cassette, 12, 13, etc. the spaces between gears are really too wide to suit me but, unfortunately, you can't make a 12 1/2 tooth cog.
At the hill climb end of the cassette, spacing that's too close can become counter productive. If the gears are spaced too closely together you'll lose all of your momentum making eensie-teensie incremental gear changes. If you have a 23 cog and a 25 cog you probably don't really want a 24.
That leaves you with the flat road cruising gears. Once you find the "sweet spot" gearing wise, it's wonderful to have an incremental "trim gear" or two in each direction so that you can adjust for minor changes in wind speed. If you're cruising "in the zone" with your 16 tooth cog and you'd like to gear down just a touch, having or not having a 17 might make a 10-speed cassette worth while to you.