Presumably for the same reason he didn't just make a Mark 3 for himself when mark 2 left with Romana, or make a Mark 5 for himself after giving Mark 4 to Sarah Jane.

He's never said what that reason is, but we can guess that either he no longer feels like a robot dog is the kind of companion he needs, or he doesn't feel like they could really replace Mark 2 in his heart. (Or, if you count the novels, his enthusiasm for traveling with K9s may have been dampened a bit when Mark 2 attempted to assassinate him under orders from Romana III.)

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