If memory serves, Randle refused to work out for Boston, which made the choice easy. You may recall that Josh Jackson refused to work out for Boston also, and that made the Tatum choice easy.
Ainge's best draft picks were not point guards. Off the top of my head, the following were his best draft choices: Tatum, Brown, Marcus Smart (can be considered a PG), Rondo (definitely a PG, but obtained via trade), Al Jefferson, Tony Allen, Jared Sullinger, Avery Bradley, Kelly Olynyk etc.
Not all great picks, by any means, but he does not only draft small point guards. Also, factor in the big moments that Smart has played in and the culture that he has injected into the team, following in the footsteps of the KG teams.
Randle may be the better all around player right now, but Smart has been contributing for winning teams since he was drafted and he has played a large role.