Or maybe it's because the champions are not that problematic or easy to rework.
Poppy was a mess and Riot was trying to figure out what to do with her. She was not an easy champion to rework and for a long time they were trying to keep her identity. It was pretty recently that they decided to actually make big impacts to champion's identities to improve their healthyness.
Garen suffers from being a straight foward champion with no room to outplay his enemies. Being extremely simple and reliable turns him into the typical Bronze Destroyer. That said, I doubt he is problematic enough to deserve a full VGU anytime soon.
Teemo suffers from a lesser case of the Aatrox syndrome. Stupidly strong when in a decent state, instant 4v5 when not. But it's not as bad. Right now he's a decent split push champion along with Trynd, Aatrox, AD Shaco and a few others. Apart from that, his entire kit is too iconic to change much.
Urgot is actually a strong champion. He doesn't have a clear cut design as other League champions do, and that's mostly his only problem. He's a single target bruiser that is pretty damn strong and has tons of 1v1 and skirmish potential, obviously falling behind in team fights due to his single target nature. In other words: his only problem is that he lacks a definite design. Aka Galio Lite.
Evelynn has always been a bit problematic, but she's a fine champion. She can go full AD or AP Assassin, she can go full Melee Carry, she can go Bruiser, she can build Tank, she can even go on hit. I truly believe this game needs more vesatile champions and Eve is a prime example. She just suffers from the classical jack of all trades, master of none problem.
Shaco is fine, just needs some fine tunning. In my opinion he's the only assassin who has to trick you into dying rather than just GETREKTNOOB100-0SKILLZ you in the face. Well, at least in the early game. To truly get ahead, a Shaco needs to take advantage of enemy jungle paths, make clever use of his boxes and think of unkown cheese strategies. Although I would argue that he should NOT be able to split push nor he should be a strong duelist and that his ult could use some hefty changes. PS: Shaco main, so I'm biased as fuck.
However, there are 2 main problems in your post:
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You ignore who is next for reworks: Warwick and Galio. Two champions in dire need of a rework. Putting anyone that you mentioned ahead of them is not that great of an idea.
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The last reworks changed about as many cheap champions as expensive ones. Take a look at the list:
Yorik (4800)
Ryze (450)
Taric (1350)
Shen (3150)
Poppy (450)
Fiora (4800)
Gangplank (3150)
Miss Fortune (3150)
Ryze (450)
Zilean (1350)
Tristana (1350)
Those were all the VGU or GU we had. Obviously not taking into consideration the pure visual ones. Only 2 of those champions were above average in price. The average of the IP price of the reworked champions is 2222 IP.
Now, if you were reffering to Rooster Updates I'm gonna agree with you, most of the big changes were made to highly priced champions. But that's mostly because they were either suffering, dominating too hard or overlapping too much. So it's understandable.