1. Even if DE doesn't "interfere" with the MC, a properly working Master-Cleanse will eliminate everything anyway ... so it's not clear how effective DE would be amid a vigorously applied cleansing regime.
2. Stanley Burroughs wrote the Master-Cleanse over 40 years ago ... and many supplements, extractions, and remedies which exist today were not around 40 years ago. To that extent, it's conceivable that a new supplement or extract would be approved by Burroughs had he known about it ... but we'll never know. DE was not commercially recognized for human consumption during Burroughs's time, so he never had the opportunity to consider its viability.