Cause snek eats toad U_U
Ok, so senjutsu is basically the absorption and balance of natural energy that occurs outside in nature, inside ones body. Too little and you will not achieve sage mode, too much and you will start turning into the animal/contracted summon. Perfect sage mode is when you have perfected the right amount to have within your body. You gain a slight physical feature change, but your body becomes significantly stronger and quicker. Also shifting into sage mode can break genjutsu.
The difference between snake sage mode and toad is that, in snake sage mode you gain a x4 speed boost whereas in toad it's x3. I could be wrong here, but in toad sage mode the users perception and reflexes are enhanced by x3, whereas snake is x4. Also with snake sage mode you gain access to Inorganic Reincarnation. This gives the user the ability to control things that have no life function, by putting some of their chakra/natural energy into it. Toad sage mode cannot do this.