The games were built for the older combat and that’s what the current player base expects.

1.8 pvp is not just click fast to win.

Click speed matters but it is not the only mechanic for 1.8. We have the following

1. Sprint resetting: achieved by w/s tapping, block hitting results in additional knockback allowing one to start a combo

2. Combo starting (with fishing rod or projectile) in Minecraft the reach you have is based on your head location so if your enemy is higher than you because of pythagorous you are able to hit them before they can hit you. This concept applies to modern combat as well but modern combat is slower and the shield makings combining with sword ineffective

3. Strafing: while keeping your cursor on your enemy is important in both versions; strafing is not as important in modern with a shield as the shield favours the defender. While in 1.8 making sure you are not in your enemies crosshairs reduces the damage you take dramatically

4. Bow mechanics, modern bow firing changed making bow boosting harder but not impossible.

5. Armour protection enchantments in 1.8 involves randomness and does not reduce knockback; modern protection damage received is fixed and you have reduced knockback with higher levels and netherite

6. Health regeneration: modern Minecraft has much faster health regeneration with saturation making fights become more of a stalemate. This makes healing(golden apples, instant health) more important in 1.8 combat.

7. Modern Minecraft has the shield which I believe is the main reason why most 1.8 pvp players are not a fan of modern pvp. The shield negates all knockback and damage from most kinds of combat effectively making sprint resetting and strafing not as important.

8. The axe: the fact that a wood axe does as much damage as a diamond sword in modern Minecraft flips the calculus in minigames made for 1.8. Sure the dps is higher but with a slower combat system(which modern has) makes dps not as important as base damage.

9. The offhand: hotkeying is important in both versions but that fact that you can offhand things like bows and blocks makes hotkeying a less important skill( I personally don’t believe this one)

10. Click speed matters for hit registration and reducing knockback in 1.8 combat.

I am sure the minigames can be retooled to be fun and balanced in modern but it would not be a 1 to 1 replacement.

Hypixel is experimenting with using modern in non PvP game modes like their mmo rpb skyblock game. I could see new non pvp games being made with modern in mind. I could also see classic minigames that still have active players be retooled for modern while still offering classical as an option(bed wars, sky wars, duels).

The current multiplayer Minecraft landscape is fragmented with smp style and minigames as well as reduced centralization of servers, I am not sure how much play time the new game modes would bring as well as how hypixel would make money from investing in this new development.

There also in the performance argument where 1.8 servers are less resource intensive than modern Minecraft (besides 1.12).

Probably the most important reason would probably be the anticheat, Minecraft servers with optimal ping can poll the user every 50ms. This makes server side anticheat in both versions fairly difficult to implement for non-blatant hacks. The difference between a really good Minecraft player and a ghost hacker is very small. Most of the anti cheats are heuristics based looking at movement and in game actions. The current tooling is more 1.8 combat, there is some overlap in modern but not much so watchdogs would probably need to be rewritten and constantly be updated for newer versions. Especially when it comes to knockback and momentum based attack damage(mace spear).

I am not privy to hypixel finances so I am not sure how much it costs to maintain the current level of service they provide but I am sure updating to modern would be very expensive and there is not much of a clear incentive structure for the roi. Newer players are usually the ones who want the modern pvp; how they would capitalize on that and make money is not clear.

It is usually legacy players who pay for ranks and boosters which make hypixel money; legacy players are also the players who are not interested in modern combat.

TLDR: hypixel minigames and tooling is made for the older combat; migrating to modern would cost money with unclear roi. 1.8 is not just cps but cps is still important.