Because Trump wants to end the war, but the US is not the country to make the call. Ukraine still has a strong military and arms industry itself and many European countries and the EU have the money and arms industry to keep Ukraine in the fight. In fact we saw that when the Republicans blocked aid to Ukraine for months. Today it is even worse for Russia as both Ukraines and the EUs arms industry have grown.

Russia on the other hand has been hit by sanctions for years, which make no mistake have hit the Russian economy hard. We are talking high inflation, problems finding money for the government, gas sales having collapsed, a somewhat likely housing crisis, defaults of lots of companies in Russia, coal industry collapsing, private bancrupcy rising fast and a lot more bad stuff. The Soviet era weapon stockpiles are running low and Russia has increasing problems finding new recruits.

Ukraine can keep the fight going for quite some time, without US help. That might be long enough to bring down Russia. The key word being might, as it depends on a lot of factors, most notably European support, but also the oil price and a lot of other factors.

Looking at this taking land has two main advantages for Putin. First of all it lowers European support for the war, as they might believe that Ukraine is going to loose, hence pushing for peace and secondly in a peace deal it gives him leverage.