The main reason why there shouldn't be challenges for 1.8 is due to the lack of gamemodes.
This leads to 2 problems:
one, players might find it boring. In my opinion the daily challenges are fun because you get to play most of the gamemodes! In 1.8 there are only 3.
Second, the daily challenges would be done probably faster. I can't be sure because I have never done it on 1.8 because it doesn't exist, but I'm quite sure it would be done quicker.
Actual gamemodes of 1.8 vs 1.9
1.8 Only: Eggwars, Skywars and FFA.
1.9 Only: Eggwars, Skywars, FFA, Duels, Lucky Islands, Minerware, and Tower defence
Overview:
This is what 1.9 challenges would look like:
Easy:
Play 3 games of minerware
kill 3 players in solo eggwars
win 3 duels
Medium:
Win a game of tower defence
Kill 8 players in FFA
Win a game of solo skywars
Get 4 kills in solo lucky islands
Hard:
Get 6 kills in team skywars
Win 40 microgames in minerware
Win a game of team skywars
kill 8 players in team lucky islands.
This is what 1.8 challenges would look like:
Easy:
Kill 3 players in solo eggwars
Break an egg in solo eggwars
shoot 25 arrows in solo skywars
Medium:
Kill 8 players in FFA
Win a game of solo skywars
Kill 5 players in team eggwars
kill 4 players in team skywars
Hard:
Win a game of team skywars
Win a game of solo eggwars
Kill 15 players in FFA
Kill 6 players in team skywars
Problem 1. (Could be boring) It could be boring because players may find 3 gamemodes not fun to play.
Problem 2. (Shorter and easier done with challenges) Normally on 1.9 it takes around 50 - 90 minutes to complete the challenges.
On 1.8 it would be shorter and easier because there are only 3 gamemodes.
If you look on the overview of 1.8, when a player wants to begin with Team Skywars, He has to win & kill 6 players. Those 2 can be done together easily.
And for solo eggwars: Kill 3 players & break an egg & win a game. Those 3 can be done in just one game.
On 1.8 it would be more efficient to do the challenges because you have to play just 3 gamemodes. Actually 5, because of solo and teams.
Then on 1.8 you would have to play around 6 games of different gamemodes to be done.
Then on 1.9 you would have to play around 11 games of different gamemodes to be done.
If I could choose, I would go for 1.8 because you play games more efficient and you are faster done.
This is not balanced compared to 1.9 because 1.8 players are done faster.
It would be easier for 1.8 to maintain their daily challenge streak, get cubelets, get exp, and get points.
I also said that faster being done with challenges is a good thing. Not for everyone, because they enjoy doing the challenges. I also enjoy them, but less since I have been doing them for over a year. I want the rewards (Mostly cubelets) so bad, that I keep doing the challenges.
Normally I don't write such long comments on suggestions, but since I play them everyday, I wanted to explain to you why I don't think it's a good idea.