They didn't really repeat shows in those days. Evil of the Daleks was the only repeated show of the sixties I think. Plus companions were coming and going a lot in those days and they weren't considered very important as can be seen by the pretty feeble exits they were getting.
Nevertheless, the usual practice was to hold the tapes until the repeat option ran out two years after first tx. You see, wipe dates for the rest of that season were...
Smugglers: pre-1970, probably 1969.
Tenth Planet: 20th October 1969
Power of the Daleks: pre 1070, probably 1969, definitely after clips were used in various documentaries in 68.
Highlanders: 9th March 1967.
Underwater Menace: 21st July 1969
Moonbase: 21st July 1969
Macra Terror: March 1974
Faceless Ones: 21st July 1969
Evil of the Daleks: August 1968, apart from part 7, 22nd September 1969.
In other words, they were all wiped after more than two years, and just as BBC1 switched from black and white to colour, apart from Evil (which was mostly wiped right after the single repeat allowed under the original contract), Macra Terror (which hung around so long that it's just possible Enterprises had already destroyed their film copies before the tapes were wiped)... and Highlanders, which went with indecent haste.
The only other story to go so quickly was Invasion of the Dinosaurs, which was authorised for wiping (though eps 2 to 6 survived) within six months, before Enterprises had even sold it abroad, and a few bits of season six (late '69, again suggesting that the key point was that BBC1 had gone over to colour).
It's just a little odd. One of those many mysteries surrounding the missing episodes.