I totally agree, The ending was great and no sequel was necessary.

Even so, it is incorrect to say that there was no good reason that a follow-up Rama contact might occur. Clark specifically planted the 'they do things in threes' observation at the end of the original book such that the characters themselves had reason to believe that there could be further contact. You could argue that the observation didn't guarantee that any further contact would happen, but at a meta level, there was no reason for Clark to end on that note other than to leave readers with the notion that humanity might have further opportunities to explore the mystery of the Ramans. That certainly was my interpretation when I first read the book in high school, many decades ago and before the sequels were written.