It has nothing to do with concentration of Test matches. More Test matches per year does not lead to more big scores in the last innings of the match.
They have more to do with quality of wickets, relative strengths of batting and bowling and the hegemony of one team in cricket in a period or a relative equality between sides.
If you look at the 400 plus scores in the 4th innings of Test matches over time, it is strange to see how they are clustered. For example there are no such scores before WW I (The first being in 1924-25 season). This is understandable since the quality of wickets improved dramatically in the 20th century.
Then there are quite a few (four in fact) between the wars and Bradman's last tour of England after the war. This period, saw some of the strongest batting in the world and not the greatest of bowling attacks (attacks mind you not individual bowlers) except for spinners like Grimmett and O'Reilly.
Then there is a huge of 25 years before it happened again.
Then with a spurt in the seventies, we have the largest gap in the two decades of 80's and 90's. Finally we have a lot of incidents again in the current decade of weak bowling attacks.
Of course there is more to it in terms of individual match situations etc. But its clear that more Tests does not mean more frequent BIG fourth innings totals.
Code:
[B]Period Tests 400+ 4th inns Tests/400[/B]
[COLOR="DarkRed"]1877 to 1924 158 0 [/COLOR]
1924-25 to 1948 147 5 29.4
[COLOR="DarkRed"]1948-49 to 1972-73 419 0 [/COLOR]
1973-1981 188 6 31.3
[COLOR="DarkRed"]1981-82 to 2001-02 694 0 [/COLOR]
2001-02 till date 339 7 48.4
Another thing to remember is that the team that is bowling in that 4th innings has to be a strong side to set up a 4th innings target of 400 plus.
Of the 18 times this has happened in Tests so far, 15 times the team setting the target has been either England or Australia.
South Africa did it once in that infamous ten day "timeless" match in 1939 and West Indies once when they had that battery of pacemen in 1974-75 against India.
The final spot is with Sri Lanka who were playing Bangladesh so could be considered a much stronger (and confident) opponent as well.