First, count the gaps, because the pattern is the clue.
1963 → 1973 : gap of 10 years
1973 → 2002 : gap of 29 years
So the first three Commissions came roughly once a decade, after each Census, and then the sequence stopped for nearly three decades. That break was deliberate, and made by a constitutional amendment — not an oversight.
The reason. Delimitation redistributes seats according to population. States whose population grew fastest would gain seats; States that had brought their population growth down would lose seats. That would have meant that a State was penalised in Parliament for succeeding at family planning. To remove that penalty, the allocation of Lok Sabha seats among the States was frozen on the basis of the 1971 Census, so no fresh Commission was needed. The freeze was later extended, and the 2002 Commission worked within it: it redrew the boundaries of constituencies inside each State using the 2001 Census figures, but did not change the number of seats allotted to each State.