"The tone frequencies, as defined by the Precise Tone Plan, are selected such that harmonics and intermodulation products will not cause an unreliable signal. No frequency is a multiple of another, the difference between any two frequencies does not equal any of the frequencies, and the sum of any two frequencies does not equal any of the frequencies."
In addition, DTMF was designed in the days of analog filters so there was consideration in ability to select out given tone frequencies with reasonable Q tank circuits.