Tradition. In the early days, horses routinely ran every 2-3 weeks.
This is how often Seabiscuit ran.
Year Starts Firsts Seconds Thirds Earnings
1940 4 2 0 1 $96,850
1939 1 0 1 0 $400
1938 11 6 4 1 $130,395
1937 15 11 2 1 $168,580
1936 23 9 1 5 $28,995
1935 35 5 7 5 $12,510
1935 was his 3 year old season I believe. Might have been his 2 year old season since he was born in 33. But he ran 35 times.
You also have to remember. The last triple crown winner was 1978. Before that 1977. Before that 1973. And the one before that was 1948 with Citation. This is how often Citation ran.
Year Starts Firsts Seconds Thirds Earnings
1951 7 3 1 2 $147,130
1950 9 2 7 0 $73,480
1948 20 19 1 0 $709,470
1947 9 8 1 0 $155,680
So Citation ran 20 times in his triple Crown year.
1930-1948, there were 7 Triple Crown winners, 2 that missed the third leg of the belmont, and 4 others that won 2 of the three legs but lost either kentucky or preakness.
But when you look at it. Since 1978, so in 36 years. 13 horses lost the third leg at the belmont. 7 others lost a different leg.