In general, blood cancers are more accessible than solid tumors. With blood cancers, the majority of the cancer cells are in the blood or in very well perfused

In general, blood cancers are more accessible than solid tumors. With blood cancers, the majority of the cancer cells are in the blood or in very well perfused tissues (bone marrow, spleen, thymus, etc), meaning that drugs/treatments can get to the targets very easily. With solid tumors, you have to deal with the complexities of the tumor microenvironment. The tumor microenvironment is the complex environment of the cancer tumor tissue. Of particular note, decreased lymphatic drainage, increased connective tissue, and aberrant angiogenesis (growth of new blood vessels) create very high interstitial fluid pressures within the tumor, limiting the ability of drugs or cells to penetrate within the tumor and "work their magic" by reaching the cells/target molecules of interest, etc. Blood cancers are a good proof-of-principle for many cancer therapies; translation to solid tumors is difficult (see challenges with CAR-T cell immunotherapy). As for gene manipulation in blood--yes, it is relatively easy to manipulate genes in blood. Hematopoetic Stem Cells (which differentiate into all different kinds of blood cells) can be withdrawn without too much difficulty, manipulated in the lab, and then reinjected into the patient.