Given that President Nixon was bold enough to erase 18 minutes of the "Nixon tapes" before turning them over to the Watergate investigators, why didn't he also erase the "smoking gun," the conversation in which he directs one of his aides to tell the FBI to back off the investigation into the Watergate break-in?
There's a good chance he thought he did.
There's actually an 18.5 minute gap in the tapes, about 3 days after the Watergate break-in. Of course that could have contained anything, including unrelated material. However, even sympathetic administration officials of the time now admit it was probably material that implicated him in the coverup of that break-in.
Now it turned out that other conversations on other tapes still implicated him. Why didn't he get those too? Perhaps he just forgot about them. There're hundreds of hours of conversation on those tapes, and this was back in an analog era when it wasn't a simple matter to compile and analyze data. It could well be that he remembered having such a conversation, he (or someone working for him) found it on the tapes, erased it, and thought they had got it all.
Ironically, the "smoking gun" tape was found only because that 18.5 minute gap looked so suspicious that Congress subpoenaed more tapes.
Technically, the gap in the tapes was caused by Nixon's secretary, Rosemary Woods. But most people consider it unlikely that she would erase part of the tapes without the direction, or at least the consent of her boss.
But here, the issue is one of the "slip between cup and lip." While Nixon is generally held responsible for the erasing of the tapes, he didn't do it himself. So then the question is, what directions did he give to his secretary. It's unlikely that he told her, "erase everything that will likely incriminate me."
Instead, it's possible that he told her to "clean up this tape," or perhaps to go around erasing tapes at "random" to confuse the issue. He would have given, and she would have followed very specific directions without knowing what was on which tape. Maybe Nixon thought that the reference to the "smoking gun" was in the 18 minute gap that the secretary erased. Maybe he created that gap for other reasons, without reference to the smoking gun.
- Nixon was notoriously a klutz, and had trouble with technology in general. If he did the erasing of the tape, it shows—multiple stop-start attempts at erasure. At least one historian I've read (sorry I don't remember who) speculates that he started by erasing that tape, then gave up, overwhelmed by the task before him.
- Nixon wanted the tapes as insurance against others' versions of the events—at least until he realised how incriminating they were when he listened to them himself.
- Nixon wanted the tapes to record his legacy, and as the basis of his memoirs. He would not have proceeded to wholesale erasure of them.
- In fact, Nixon multiple times mused that he should destroy all the tapes. He never went ahead with it.