As others have said, maximum container ship dimensions are constrained by canals, ports and terminals. Ports and terminals in particular have, over the last few years, had to spend huge sums of money on enhancing and enlarging approach channels, turning circles, berths (length and depth), gantry cranes (outreach and height). The ever larger ships offer economies of scale to shipping lines, but push up costs for ports and terminals (and offer no more cargo than before, just greater peaks of cargo).
It seems though, that for the time being at least, the lines have realised that they can't push beyond the "400m long x 61m wide x 16.5m draft" footprint.
There is a commercial consideration though as well - if lines go for even larger ships, they have to find enough cargo to fill them. Today, the only way lines can fill 20,000 teu ships is by operating in alliances, and we are already down to just 3 alliances.