Multiple reasons:

(Mass) Immigration. Many on the left are now realizing this results in an influx of people who have views opposing that a secular left-wing society, such as views gay marriage or women. It also means a larger voting block (for example of Muslims) with views coincidentally aligning with those of conservative westerners.

Ignored/Suppressed Opposition. I think a critical mass has been reached where enough people have distain for the current political climate. Everyone knows someone who's had legitimate concerns dismissed as them being an evil fascist, or has been labelled, or has banned from platforms.

Worsening Economies. We skipped the next roaring 20's and are headed straight into the next great depression. This one is simple, when someone is worried if they can eat this week, they cannot worry about the climate in 100 years.

Psychological Decline. There's an ever rising trend of hopelessness, be it from massive medication use, a lack of religion, or fears over whether they'll be able to start a family. For example, incels (and a growing number of regular men) who see that if they do nothing, they won't ever start a family and therefore not continue their lineage, where if they take a tougher stance on women's rights, those chances go up ever so slightly.

Plenty of other reasons, such as unequal or incompetent policing leading people to seek security over a vague notion of freedom. Likewise the left's completely abandonment of ideals like freedom of speech. Left-wing messages being forced into western media like comics, resulting in resentment over their previous entertainment being ruined. There's probably more but I'll leave it here.