Why has hashem given us a mitsva without a reason and one which is impossible to understand. All the others even the 'chukim' at least Solomon understood. This one even he didnt that shows it is impossible to understand the reason. Although moshe rabainu did he was a 'special' case. Why have mitsvot where one can never know the reason. The mitsva of poroh aduma is the only one where some on earth with his limited 'sechel' cannot understand. All the others can be. So the question is since all mitvot can be understood as King Solomon has proved, and that is really are purpose in life to try to understand them, why have one which cannot be understood.
The way I understand it is that mitzvos which are chukim show our true belief in Hashem. Mitzvos that we do understand one can say we are doing them because they make sense to us but if they didn't make sense we wouldn't do them.We as Jews treat all mitzvos alike whether we know the reasons or have the slightest clue , we keep them no matter what and for that reason we have many mitzvos which are truly unknown to see who believes and trusts in Hashem.(I have seen this reason brought down in a sefer ,don't remember where.