@yalanland. It is the power of the EUV light source that is the limiting factor. A couple of different types of source have been tried. As well as getting the source to shine that bright I believe that reliability and uptime is an issue.
If you go brighter but have to take the machine down to faff around with the source for large periods of time the wafer throughput you gain during the uptime you more than lose in the downtime.