I feel like a majority of the Div 1 problems on Codeforces have $$$n \leq 10^5$$$ or $$$n \leq 10^6$$$ here, which basically means that the solutions have to be between $$$O(n)$$$ and $$$O(n \log n)$$$ (maybe in rare cases $$$O(n \sqrt n)$$$ or $$$O(n \log^2 n)$$$). However, this doesn't seem to be the case in other high level competitions; for example, this rarely seems to be the case in Topcoder Div 1 SRM, Google Code Jam, and ICPC.

With such a high constraint on $$$n$$$, the solution space is pretty constrained, and only certain algorithms will be viable. This might explain why certain data structures like Segment Trees/Fenwick Trees are overrepresented on Codeforces compared to other platforms. Other interesting algorithms like Max Flow, Knuth DP, Blossom Algorithm, etc pretty much never appear on Codeforces, and when there is a difficult problem (eg Div 1 C/D) with constraints that are not $$$10^5$$$/$$$10^6$$$, you can pretty much expect that the solution will be one of these algorithms that rarely appears, as opposed to other platforms where you would have to be on your toes for all problems.

I have never formally set problems before, so perhaps there is something I am missing (eg perhaps $$$n \leq 10^5$$$ are easier to test and block incorrect solutions), but is there any particular reason that there are so many problems with this constraint? I feel like contests would be filled with a more rich variety of algorithms if this was not the case.