An important year for Doctor Who, 1973 is the 10th Anniversary. Strictly speaking the series won't have been on the air for 10 years until November 1973, but since the series now never airs in November, the anniversary story "The Three Doctors" kicks off Season 10 11 months earlier on the 30th of December 1972.
Well, as I've pointed out before, the vast majority of viewers are not strictly Doctor Who fans, so the above paragraph wouldn't mean a great deal to most of them. And this is reflected in the good but rather ordinary viewing figures. The season starts well at 9.6 million, peaking at 11.9 for the finale of "The Three Doctors", which would seem to indicate the chance to see Hartnell and Troughton did pique the interest of a significant number of viewers. However things settle down almost immediately afterwards to a quite respectable 8-9 million. The figures droop a bit over the next few weeks, before jumping back up to over 10 million for the much-heralded return of the Daleks.