Why you can't summon face-up monsters in defense position?
I know it is the rule you can only summon in attack position or set (face-down defense position)


To correct on your phrase, one can only Normal Summon in Face-up attack position.

Unless otherwise restricted, you can Special Summon in Face-up defense position.



The actual reason for doing so I guess is something only Konami knows. If I were to conjecture on the reason, I would say that it is because not only is your monster in defense, which could be tactical advantage, but you will also be able to activate its effects. 

To prevent this abuse, the rules force you to Normal summon in Attack position (possibly a disadvantage, or at least open for battle damage) if you wish to activate your monster's effect. Think of it as "if you want to activate this effect you have to take some risks".

On the other hand, I also think this could be a "Legacy" rule, one that has lost its original impact, as now there are many ways to summon in face-up defense position (one barely Normal summons now days). Perhaps Konami decided to let it be, to prevent possible abuse.