I think that everyone has given great answers. It's really pretty simple -- Disneyland Resort has gotten more popular and crowded, overall, because more things have been added; there are more things to do; more people have Annual Passes; more people are making DLR a destination when they used to go to WDW; and Disney is adding in more events (I call them "pop-up events") to fill in the less busy times.
Prior to 2006, Halloween Time (the season, not the specific Halloween party) was not a thing. Haunted Mansion Holiday had been around for several years, and the Halloween party actually started back in the '90s before being shelved, but when those things suddenly became packaged as part of Halloween Time, crowds grew. The Halloween Time season became official in 2006, and it bridged the gap between the super-busy summer season and the super-busy holiday season, eventually becoming a big draw of its own.
Technically, by Disney's standards and numbers, most of the Halloween Time season is still considered to be the off season -- but from a guest's perspective, it doesn't appear to be off season at all because there is no shortage of people in the parks.
In the late winter through late spring, now there are things like Bayou Bash (which skipped last year due to, I think, the work that was being done on Club 33) and the Easter egg hunt (I saw a lot of people wandering around both parks with the maps for the hunt in their hands last year), and the 24-hour event, etc. Of course, Frozen Fun is popular. This year (and probably into next year) there will be various events for the 60th anniversary. Within the next couple of years we will probably see a lot more changes in both parks (I think that DCA may be teetering on the brink of another extended phase of significant changes, beginning with the re-imagining of Condor Flats, and Disneyland will probably get some sort of Star Wars something-or-another added in).
There's always something going on now. I think that the only time there would truly be a significant off season anymore is when there is literally nothing out of the ordinary going on -- no special holidays of any kind, no special "days" (Bats Day, Dapper Day, Gay Days, Raver Day, whatever kind of day, etc.), no festive seasonal events and no new attractions or shiny new versions of old attractions.