This is a question you need to ask the patient's doctor. I can give you some ideas but only the patient's doctor can confirm the reason...
1) A patient suffering from low platelets, depending on the cause, may do best with a medication or treatment other than blood. If this is the case, it could be dangerous to give blood, even if it is from a relative.
2) Blood relatives do not always have compatible blood. The blood relative(s) who want to give may not have the correct blood type or their may be some issues with antigens that make an adverse reaction dangerous. If this is the case, a transfusion from a non-relative with compatible blood is safer and necessary.
3) The patient may need something other than whole blood. If blood is mostly within normal limits or the patient has certain health problems, it may be best to correct the abnormality by giving a specific part of blood -- for example, red cells, plasma, platelets, and clotting factors can be given individually. These products can be removed from whole blood to be given on their own. Some of these products are created by pooling the blood of many people and extracting the necessary part(s). Multiple donors may be required to collect a sufficient amount of whatever blood product is needed.
4) In an emergency, there may not be enough time to collect, test, and process blood from a blood relative. If the platelets are low enough to be life threatening, blood or blood products may need to be given immediately.
I hope this is helpful.