I´ve been able to find an interesting article on the subject.. "for those with little knowledge of Spartan History, this should help. And for those with more knowledge, it will backup some of the points that we have already raised"

But, I still haven´t been able to find a concrete reason as to why Phillip didn´t just invade Sparta, when I say concrete I mean a clear and definable reason for the situation we are discussing.

In my opinion, I think that we can´t be too far away from the truth !
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"After the battle of Chaeronea (338 BC) Phillip of Macedon marched through the Peloponnese, welcomed by all the cities but when he reached Sparta they refused him to enter. Phillip did not try to take the city by force and left. Sparta was the only Greek city that did not take part in the League of Corinth, which was formed in 337 BC, under Macedonian control.

In 272 BC, king Pyrros of Epeiros could easily have taken the city after defeating the Spartans. Sparta became a dependency of Macedon, regained independence under the tyrants Machanidas (207 BC) and Nabis (195 - 192 BC).
In 265 BC again, having formed an alliance with Athens, Achaea and Elis and some Arcadian cities, gave battle against Macedon but lost it and in his retreat was killed (Chremonidean war).
The son of Areus, Akrotatos, in 260 BC leading the Spartan army against Megalopolitans, he was defeated and himself killed.
In 244 BC, Agis IV came to the throne and starting a series of changes. He proposed all debts to be cancelled, and to redistribute all land, in parts of 4500 citizens and 15000 Perioikoi. He also insisted on strict Lykurgian training in the citizens for the remained 700 equals (omioi) and 2000 hypomeiones and selected perioikoi. He found in his proposals strong resistance and Agis was put in trial and executed in 241 BC."