Because the Government is broken with it. The number of filibusters each year is getting ridiculous. Yea, Democrats should be able to filibuster bullshit legislation, but it's gotten to the point where without having 60 votes in the Senate you cannot pass anything. There is no way Trump doesn't tank the Republicans in 2018 and 2020, and when the smoke clears Democrats stand to have a majority hold on the Government, maybe not a super majority, but a majority. If we let the filibuster die now, when 2020 rolls around the Democrats can actually get something done, and probably reverse all the damage Trump did within a few months or a year.
The need for 60 in the Senate leads to a "must win" attitude in Washington, and that needs to go. Nobody gives a shit about passing legislation, it's about taking political cheap shots to ensure you get re-elected. If we didn't have the filibuster, the excuse of "but it will never pass/it will just get filibustered" no longer exists.