Because the monk doesn't do any of them well.
I disagree. I think the monk does do them well.
For example, Unarmored Defense. by the rules a starting monk with 16 Dex and 14 Wis only has an AC of 15 and has to wait until level4 to get to 16 AC.
Or Ki abilities. The monk lack enough ki to do them often. So either they need more k or the monk needs to be able to do them for free at a weakened capacity.
Or Improved Movement speed. The Monk is slower than a Rogue because the Monk relies on Ki to move fast.
AC 15 in tier 1 is decent. A rogue with DEX 16 will also have AC 15. A greatweapon fighter with heavy armor is likely AC 16, etc.
The monk is right there with them. Now, what I think is the DEX + WIS model should be replaced with DEX + WIS + Proficiency bonus (or the best two of those three, removing a little of the MAD for monks).
Ki is adequate IME, but I understand why many people feel they need more ki, and adding WIS mod ki points solves that issue IMO.
The +10 speed of monks at 2nd level and beyond is all the time. Rogues
can Dash each round, but at the expense of other bonus actions. The extra movement of monks is constant and goes to +30 in the end.
The monk really should have a column for bonus AC and bonus attacks. And the column for bonus movement should scale faster.
Either replacing DEX or WIS with proficiency bonus would allow a Monk to get AC 21 in tier 4. Using all three might be too strong, but will remove some of the MAD dependency as I said.
Bonus attacks are already there. Adding a bit more ki allows more uses, which is sufficient to me. Even without it, at level 5 monks can get three attacks each round anyway.