Robert Hooke observed the cells through a combination of lenses.
After Robert Hooke, the development of microscopy take place. After the development of the microscopy, scientist were able to observe the features of the cell. In order to formulate a cell theory observation of most of the cells was necessary. So, it was necessary to observe different types of cells from different organism. So, the advancement of the microscopy and observation process required for the formulation of the cell theory was the reason for the delayed development of the cell theory.