Because in roulette, winning on red or black is actually a 18/38 bet. And over ten spins, you are likely to be about even, probably behind 1 betting unit. Over ten spins on a single number, you are more likely to be ahead because you've got a little under a 10/38 chance of being ahead, probably about 8/38 - since numbers can repeat.
If you win at least one of the spins, you'll be ahead if you don't keep on playing. If you bet on a single color, then you're going to be more likely to win SOME of your money back, but still be even or behind. Obviously you could get 6 or more spins right, but it's more much more probable that you got 5 or less right.
Which is why I said you're more likely to end up far ahead betting on a single number than betting on a single even money bet - that's what volatility describes. There's a house edge, so don't mistake that for saying you're likely to come out ahead playing a single number in roulette.
Over 3800 roulette spins, you should see 1800 red, 1800 blacks, and 200 greens - or close to it.
Over those same 3800 spins, however, you'd see a bigger variation in specific numbers' frequencies, (even though they should be relatively close to the average, the less likely the probability of an event, the bigger the necessary sample to before it should pan out to about it's average frequency). The range of frequencies would likely be between 70-130 shows each. Although, I haven't run any specific simulation or seen any, so that's an estimate I just made up, but I know you know what I'm trying to explain.
Over a lesser amount of spins, say 38 spins, a player betting on even money bets can expect to win about 18 spins and lose 20. There'd be some variation from this, let's just say that you'll probably win between 10-26 spins out of the 38. But a player betting on one number can expect to win between 0 and 3 times. If we both made a flat even 10 dollar bet over twenty spins, me on a single digit, and you on an even money bet, say you won 26 of the spins - way more than average (that's a very small chance, like probably less than 1 percent), and I won 2, which isn't the most likely scenario, but a decent shot, then you'd have won 260-120=140 dollars profit, while I would have won 720-360=360.
In this scenario, I was more likely to lose all of my spins than you were, but you more likely to end up behind. This is because even though have to beat lower odds to come out ahead - you have to do it multiple times - you've got to beat the 18/38 odds more than half the time. In the scenario I wrote out you'd really likely get about 17,18, or 19 right, and I'd most likely get 0 or 1, I've got a higher probability of coming out ahead than you and I've also got a higher probability of losing every spin.
If I beat the 1/38 odds once within the first 10 spins, I'd come out ahead. For 38 spins I'd have to win at least twice. I'm more likely to get nothing, but also more likely to come out ahead, because if I win, I win a large amount.