pericynthion is correct that the pacing item for the moon landing was LM weight.
In the case of ascent stage failure, CSM had fuel to maneuver to meet the LM. As pericynthion notes, this is an unlikely failure mode.
In the case of failure to dock, the LM crew would EVA to the CSM -- not a maneuver you'd want to undertake if you had a better option, but not all that risky.
As it happens, making Apollo 10 the landing was considered:
According to multiple sources, some consideration was given to assigning the first lunar landing to Apollo 10, which [Tom] Stafford commanded. It was ultimately decided that Apollo 10 could land no more than one month sooner than Apollo 11 could (because Apollo 11's lighter LM was required, and because the Apollo 10 crew would have needed some additional training), and that the additional tracking experience from a second lunar mission would be useful when planning the first landing. Apollo 10 therefore flew a lunar orbital mission as originally planned. Stafford later flew again as CDR of the ASTP mission, his fourth mission.
I can't find the reference, but I believe Stafford himself felt it would be better to make A10 a dress rehearsal flight rather than the first landing.
It's important to remember that Apollo 10 was only the second translunar flight ever, the second manned flight of the LM, and the third manned flight of the Saturn V. Every Apollo mission had had some anomalies, and it was by no means clear that the complete system was reliable. Another opportunity to work out the bugs was welcome.