Haisman and Lincoln were very unhappy about the way the production of The Dominators went: there were problems over the copyright for the Quarks (which they hoped would be as big as the Daleks and make their fortune, and which may have been licenced to TV Comic without their OK), and the script editor (disliking what he was getting) cancelled their six episode commission after receipt of episode four and wrote his own single episode conclusion (plus an extra filler episode stuck onto the front of The Mind Robber).*
They took their name off the serial, and started legal action to try and prevent its transmission (which may be why a complete set of prints has always been kept by the BBC, as evidence related to the case. That's pure speculation, but it would explain why The Dominators was at one point the only complete story form the black and white years in the archives apart from An Unearthly Child).
The case was eventually settled out of court and the serial shown as planned, but the pair never wrote for the series again, with the third Yeti story that was already in the works being dropped as a result.
*Edit: Actually, I think he cancelled after getting episode five,m at which point he re-edited 3&4 into one episode and then wrote his own one episode conclusion. But it stil ended up as a five parter rather than the six originally commissioned.