Why is Tuvok so uninspiring or uncharismatic compared to Spock?

Disagree with your premise. I thought Tuvok was great and loved the episodes where he was the lead character.

The simple answer is that Tuvok is so much more mature than Spock, that the Trek writers didn't know what to do with him. Really!

Spock may have been the smartest person in Starfleet and a total badass, but emotionally he was still very much an adolescent - trying to figure out how to balance emotion and reason, fighting with his parents over career plans, and struggling with the onset of hormonal urges. The geeky Trek writers could get into that mix of fantasy fulfillment and immaturity! Conversely, Tuvok was conceived as a mature, stable, unconflicted adult, someone whose family, friendships, and belief system are all free of conflict. There were no flaws or drama built into his character, and IMHO Russ never found a way to rise above the written-in limitations he had to work with and to really connect with the audience.

So the writers for "Voyager" just had Tuvok get hurt over and over again, and let us watch him suffer instead of growing or changing as a character. It was repetitive, it got old, and was a real failure on the part of the show.

The episode where Tuvok explained why Vulcans surpress their emotions really opened up the Tuvok character for me. It also did a good job expanding the Vulcan race for the Star Trek universe. I believe this was the episode

http://memory-alpha.wikia.com/wiki/Gravity_(episode)

Come on, just another example of the black characters on the show getting sidelined. They did not write for him because he was like a Sulu or Checkov character, a moving cardboard cutout.

As I said above, one problem is that the writers really didn't know what to do with a Star Trek character who is mature, responsible, dedicated to duty, and not fucked up.

But another factor is that Leonard Nimoy just did more with his Vulcan character than Tim Russ. Yes, Nimoy had the whole half-human thing to work with, but the fact is he managed to make Spock interesting and oddly likeable, he had a great deadpan humor, occasional pathos, and a deep capacity for love that underlay everything; in his last appearance in the role in 2009 Spock had matured into a real Bodhisattva. Tuvok, on the other hand, was played as uptight and unhappy at baseline, not someone you really wanted to get to know better.

I just read this thread and a lot of good points were made. I think having Shatner and Kelley on his wing helped immensely, and Nimoy was a veteran actor already, (as the trio all were, I believe). Plus LN was an interesting looking fellow, tall and quite thin, and his eyes always seemed a little Asian to me. Sometimes I'll see him in an old Western and I can't help but think of Spock, haha. Leonard just had a little more charisma than Tim Russ, and likely better writers and fewer other characters taking up screen time.

Tuvok is my favorite Vulcan. He was more assertive and didn't take BS from anybody.

I guess one couldn't expect every Vulcan to follow suit. Spock after all was part human and part Vulcan and probably had insights and ways of communicating that expressed Vulcan intellect quite vividly to his human colleagues. Leonard Nimoy himself had a unique interpretation that he brought to the character that brought depth and meaning to it.

Another reason may be that the writers wanted to look into the Borg further and create a new "rational alien" in the form of 7 of 9 that was to become a close friend of the Captain character on the ship. This was a fresh idea and one in which they could involve the character expressing emotions somewhat freely even though it was from a dispassionate personality.

So even though the Vulcans are a notable culture and they always seem to contribute admirably to Starfleet in whatever capacity, they seem to be treated as secondary characters in the writing of the show.