Famously, they didn't support Windows for the first decade of their existence, and that did cost them a tremendous amount of market share against MySQL.
By their technical merits, PostgreSQL should be the most used DB.
I've followed the project since the 6.x days and I am still impressed that they managed (with the help of some really sharp win32 people) to get their fork-per-client model working on Windows.
yep. the windoze process model very much not like the unix process model. pg has been happily unix since 6.x days.
also, certain key players in pg community have threatened to quit if pg went the threaded execution model. getting threading correct is very, very difficult.
took many years (with much help from win experts) to get the port correct AND maintain the unix process-per-query model on unix.
By their technical merits, PostgreSQL should be the most used DB.