Still, look at who Shatner had most of his scenes with.
Most of the time actors spend on a set is not spent actually performing. It's sitting around waiting and waiting for the next setup. As such, you are likely to bond with anybody who is physically on the set, as long as you don't have a snobby attitude about who is or isn't worth talking to.
I realize that, but for the most part it seems the big guys went back to their trailers during set ups, unless they were at the "rehearsal table" instituted by Marc Daniels. And since Shatner and Takei didn't have that many scenes together in comparison to scenes with Nimoy or Kelley, he'd still be spending more time between set ups with the main cast. Takei and the others wouldn't even be on the call sheet to pal around with. He didn't have to be a jerk, but neither did he have to know a damned thing about Takei's life. He admitted in his first book that he realized, after 25 years, he never really knew the man.
Honestly, if someone in my life made it that obvious he didn't care to know me, he'd he off my radar except when we had to work together. When asked, all you'd get would be "a nice enough guy, but we weren't close." It all adds up to the same thing for the lower tier cast: get over it.