Why do we need to wait to upgrade Smite?

Because Riot wants to screw jungler. That's not how they'ld phrase it though. But they said that they "intend some junglers to back to base at level 2 or 3".

Why can't you get Homeguards on Boots of Speed? Because that maintains the pacing of the game. Remember that most of what Riot has been doing is preventing unstoppable snowballs and balancing the gameplay so that any two team compositions have a relatively similar chance on winning, so it's more skill and decision making based, rather than "I got items before you" game. Albeit, getting items first helps you snowball, but you must have farmed more efficiently, acted in some plays to get some K and A, or secured some objectives for the local gold. A slow farmer like Shen would be outclassed, and Riot wants to expand the viability of junglers, rather than locking it down to the few like Lee Sin and Elise:P

Then Smite top/bot/mid would become viable. To get the upgrades as soon as possible, don't be a lazy ass and keep farming jungle monsters.