Because he did an extremely unpopular thing. I don't recall if it was here, or in a news article I read, but I saw a poll that indicated that the Dem constituency overwhelmingly wants the Dems to fight. No appeasement. Just fight.

Of course. And Schumer didn't do that, because he knows more about how politics actually works than the "Dem constituency."

It makes things worse if you fight in the wrong spots. To take a recent example, progressives thought that Al Green's protest and the various little signs and protest stickers at the Joint Session were a worthwhile "fight" to bring to Trump. I'm sure they were correct in assessing that their "Dem constituency" preferred those performative objections to normalizing Trump by sitting there (and maybe cheering the kid with cancer). But that "fight" didn't actually do anything. It wasn't a fight. It didn't affect Trump at all. It didn't make the Democrats look firm or strong (I didn't see anyone on the Democratic side think that was a successful outcome). It didn't energize the Dem constituency, because it failed.

So if Schumer had voted down the CR, it wouldn't have been fighting - it would have just been failure. The government would shut down for 5-31 days or so, the GOP would not bargain, and the Democrats would eventually just have to approve the same CR anyway. Mostly because no one ever can use a shutdown as leverage. It never works. It didn't work for Gingrich, it didn't work when Cruz wanted to lever the shutdown into defunding Obamacare, it didn't work for the GOP trying to get border wall funding. Voters don't like it when the government shuts down, they don't want anyone trying to use it to gain leverage on policy matters.

Here, it's worse - because the shutdown helps the GOP. The Democrats would be "fighting" Trump the same way that Brer Bear and Brer Fox "fought" Brer Rabbit by tossing him in the briar patch. It's a good outcome for him. Once they passed the CR through the House, it would have been better for Trump to have the shutdown. All of the chaos in the federal government would get mixed in with the chaos of the shutdown, he would get to furlough all the employees he wants to fire instead of having to put them on administrative leave, more of them will have to resign because they're not getting paid, and if he can get the shutdown to last 30 days he basically gains massive new powers to fire tons of people. And eventually the Democrats would have had to vote for the CR anyway....which would just disillusion the Dem constituency even more than they're upset now.

It's just magical thinking. The Dem constituency wants to fight because they foolishly (and mistakenly) think they would win. But they wouldn't. Schumer knows it - he knows his members would not have been able to withstand the heat from being blamed for the shutdown. And while the Dem constituency thinks they'd be happier having fought and lost, that's not usually what happens - when you fight and it makes the other guy better off you end up feeling worse. They only reason they want to fight is because they're deluding themselves into thinking it would end with anything different than the same CR but Trump stronger for having the government shut down for a while. And they're wrong.