The gemara in Shabbos comments that Hashem held the mountain over the Jewish people's head and forced them to accept the Torah. The obvious question is that they said נעשה ונשמע. The acharonim point out that the medrash in Noach asks this question and answers that they said נעשה ונשמע on תורה שבכתב but they had to be forced to accept תורה שבעל פה because it is infinite and hard to master. It was only at the time of Purim where the Jews were mekabel תורה שבעל פה out of their own free will (קימו וקיבלו) and this is why the flowering of the תורה שבעל פה only came about in the time of the second beis hamikdash.
RYBS explained the idea of kedushas haaretz based on this. He explained that all kedusha stems from Torah, it is the only source of kedusha. If so, where does kedushas haaretz come from. He explained that when Bnei Yisrael conquered Israel they sent the Aron in front of them which actually won the battles, the Aron represents תורה שבכתב and therefore the land was niskadesh with the kedusha based on תורה שבכתב.
Based on this he explained why kedusha rishona didn't last while kedusha shniya lasts until today.
The Rambam already gives a cryptic explanation, that the first kedusha was based on כיבוש and therefore since the land was conquered from them it was destroyed while the second kedusha was based on חזקה. However, this is not easily understood why should this be so.
The Rav explained as follows. The first kedusha was based on תורה שבכתב which is finite and physical and therefore just like the תורה שבכתב can be destroyed so too the kedusha can be destroyed. The second kedusha on the other hand was kedusha b'peh, based on תורה שבעל פה which is not physical and infinite and therefore the kedusha it produces cannot be destroyed. The first kedusha had to be based on תורה שבכתב because that was all they had, they had not accepted תורה שבעל פה out of their own free will. Only at the time of the second Beis Hamikdash after they had already accepted תורה שבעל פה freely could they use it's power to be mekadesh the land.