Why can't different current sources connect in series?
It is a basic rule. The physical basis for it is as follows. Current is just moving charges. Charges cannot accumulate. That is, objects generally cannot acquire a net charge. That is both a physical rule (rule of the universe) and also a rule for circuit analysis. So every time charges move into one side of a conductor, an equal number of charges has to move out on the other side.

So if I2 is dumping charge into the conductor at 10A, and I1 is clearing charge from the same conductor at 5A, that would imply that charge is accumulating in the conductor. Since that is impossible, it is impossible for two current sources to be in series unless they have the same current.

I hope you find this to be a satisfactory answer. I have tried to be accurate but intuitive. There are lots of analogies that could apply also (cars on a freeway, or skiers getting on a ski-lift or something). Let me know if you want me to offer something of that nature.