I'm skeptical this was the reason, though (at least in and of itself). Going off of https://en.wikipedia.org/wiki/Multics#Novel_ideas, it seems that the address space (36 address bits ⇒ 36 bits/word × 2³⁶ ÷ 8 bits/byte = 288 GiB) was much larger than memory of the machine Multics was designed for (about 2 MB), and the size of a segment was limited to about 1 MB (36 bits/word × 2¹⁸ ÷ 8 bits/byte), nearly half of all memory – so I doubt this was troublesome at the time.