According to Palestinian sources, this decision is due to Qatar's disappointment in the slow progress of infrastructure projects on the ground.
Since October, Qatar has given around $10 million a month to Gaza for fuel to run it's power station, one of three sources of electricity in Gaza. The other two sources are gas lines from Egypt and Israel. Because of damages to the pipelines from Egypt, Gaza has not received electricity through that source since February 2018.
The Qatari funding has enabled Gazans to have up to 12 hours of electricity a day, whereas previously, in late 2018, there would often be as little as four hours of electricity a day.
Qatar has also been transferring $15 million dollars a month since November to pay for salaries for civil servants in Gaza. These payments are set to end in May.