If I normalize to look at your portfolio as "Years of salary saved" it would be obvious that early on, you are going from zero at the start to "1" perhaps 5-8 years in if you are saving a decent percent, 10-15.
At "1" if your desired mix is 40/30/30, but you are at 50/30/20, without depositing to the "50" asset, you'd need to get the other 2 up to 37.5%. Since the total assets are a year's salary, you can see how quickly the misalignment might take years to resolve. Here, deposits totaling 25% of a year's income.
Let's me offer a differ way to look at it. Say I wanted an 80/20 balance, S&P/cash. The S&P has been on a tear since the bottom, tripling in the last 9-10 years. Using deposit to balance might mean never depositing to the S&P again. This would also mean losing any potential gains for the 'dollar cost averaging affect'.