To put this in perspective, Sophocles wrote over 100 plays. Someone thought that seven of his plays had value, and were subsequently preserved. This one example has been repeated throughout history.
Well, others were preserved at various times, and subsequently lost at various times. For instance, the Library at Alexandria preserved official copies -- or even originals -- of the plays of Sophocles, Aeschylus, and Euripides, and considered them to be of such value that they were reluctant even to loan copies out to anyone. But not everyone values the same things, and those plays and a wealth of other knowledge were lost when the Christians took over Alexandria and burned the library's contents in their purges of pagan culture. There may have been other copies and collections that have been preserved and lost at various times; but those seven happened to be part of a compilation that existed recently enough to have survived to the present.
And so it is with the missing Doctor Who
episodes. Which ones survived the purges and which ones didn't was more a matter of chance than deliberate selection.