Angry birds says hello.  But seriously, most games today focus too much on technology and realism that other aspects of character design like aesthetics, sound cues, and other endearing qualities get little to no attention.  They need to go back to the roots when designing characters, especially the main characters.

Personality. He has none. Its just a moving sack.

Also, because of the customisation options, he doesnt have a strong, fix design either.

I love Sackboy but as the number one mascot I think not, He is way to kiddy and would weaken Sony's hardcore image. I prefer Drake of Kratos as the face of Sony

IDK...but it was the greatest console exclusive platformer of last gen imo. LBP2 was just, yes.

Really i always thought that Crash Bandicoot would be Sony mascot until activision destroy it.

He's too weird ... "Sackboy" isn't really that cute, he's a little creepy looking (just a bit anyway).

The LBP games were OK but not really that mind blowing. The level design was simplistic and the controls were floaty.

The create-a-level stuff was neat, but people tend to want to play platformers for a quick fix of fun, not something to spend hours and hours in.

Probably would've done well if Nintendo had pushed it in the Wii days, but that's out of Sony's comfort zone.

(1) The LBP games really aren't that fantastic. Sony fans praise them, yeah, but for the rest of the gaming community, they're very hit-or-miss. I bought LBP2 and LBP Racing and was very underwhelmed by both.

(2) Sackboy has zero personality. The very premise behind him - being fully customizable and neutral - is precisely why he's not mascot-worthy. One could say similar things about Miis, but they're only fully customizable in regards to appearance (and really, only facially). They have tons of personality, annoying as it may be.

If (1) the LBP games played more smoothly and (2) at least the main Sackboy was more memorable a character, it'd have taken off more.

Sackboy didn't eat any mushrooms. Also because LBP is an exclusive.

Because he does not appeal to most of sony's target audience and secondly because he has not had enough time compared to other platformer characters.
I liked LBP, but I only played them because the first came bundled with the ps3 and second was from PS+. I dont think I would have gone out and bought them.

I also think the fact that they said they wanted LBP1 to last the entire PS3 gen killed the hype for the next one which they still released on PS3 anyway.

Perhaps because it was one of the most uninspired videogame characters to ever exist with one of the dumbest names in history. Sackboy sounds like an insult among homophobic 10 year olds.