One reason is as it says in the Petek, "with the strengthening of your devotion you will understand it", even Saba didn't have a proper understanding of the Petek until he strengthened his devotion, ultimately Saba merited to have amazing understanding of the Petek, proclaiming that if he would reveal what he understood about just two word of the Petek he would take away a persons free choice, and even at this height of understanding Saba said he just understands but a fragment, like a drop in the ocean, for what can we understand of Our Master Rabbi Nachman. After 60 years Saba was inspired and confident in his understanding to start spreading the light of the Petek.
Another reason why Saba withheld the Petek from the general public was his fear of the terrible influence and scoffing of some of the MSS - the honor and power seeking rabbis in Breslov that were not only jealous of Saba having received the Petek, but were full of venom and hatred for what could undermine all the stature they had worked so hard to build. It is well known that the real Breslovers accepted and revered the Petek, these included the elders, Rabbi Naftoli and R' Shlomo Wexler, R' Yitzchok Breiter, and R' Shmuel Horowitz, but those that had built for themselves a following and positions of power felt very threatened by Saba and the Petek. It is amazing to note the Hashgucha - Divine Providence, that one of these died on Chag HaPetek 59 years after the receiving of the Petek (1981)! Another one died on the eve of Chag HaPetek (Saba actually received the Petek sometime at the beginning of the night of 23rd of Tamuz) in 1989!
Saba said "we don't need the rabbis, the nation itself will choose Rabbainu"!
Great blessings of Na Nach Nachma Nachman MeUman!