All the other horcruxes were hidden and guarded carefully and with a lot of thought as to where they were hidden. It seems odd to me that the diary was given to the someone to keep safe. Even if he needed someone to use it to open the chamber of secrets ... It still seems a big risk.
Why was the diary given/entrusted to Malfoy Snr? Voldemort, who works alone, trusts something such as his diary horcrux to a person to keep safe. That seems to be a bit odd to me.