Why and how to change this ?

If you want players to stick after boost ends.
1- make it balanced <-
2- the more the server is noob friendly the more you will get noobs. X- you will lose them after boost ends.
3- choose good maps.
4- lower the server resources if you want if you want more players to come from boost.
5- use week boosts instead of normal boosts if your server isn't easy for noobs.
6- use maps where humans can solo zombies for long time, take ex : zm_cubeworld_v1

no way that what you say is correct.
1- kgb supports rehlds = reauthchecker without a single edit on files
2- people using Kgb servers boost and get players as any other servers
3- headless chickens aren't bots. they are worse

bro can explain to me what is Re-hlds ?

ReHLDS was created by DreamStalker not VALVE, it's a voluntary work.

mr. ZP

2009-2011 CS 1.6 was the top shit, everyone was addicted to it, even me, I'd spend the entire night playing Zombie Plague and there would be 31 other idiots that would play all night.

2012-2014 CS 1.6 started to fade away, the new generation started playing other games and CSGO came out.

2016-2018 CS 1.6 is almost dead, and it's dying day by day. Sad but truth reality.

So you shouldn't really expect a lot of people anymore since there's not a lot left.

What affects the player number:
1) You should upgrade your player slots from 21 to 32, I know it sounds dumb but a lot of people think like this 'the more slots, the more the chance to have good stuff'.
2) Boost only at the period of time where there actually is online people, so around 8 to 10 PM.
3) Don't add too much resources if you don't have a really good FD. Other than the downloading time, having too many resources lag the players.
4) Choose your community wisely, toxic admins can cause a lot of players to quit.
5) Having active friends means having regular players, a lot of servers don't even need boosts because they have regular players.
6) Consider creating a wide friendship with a lot of players and posting in social medias.
7) Give a lot of rewards to players, so they can actually feel like they're being rewarded.
8) Free VIP at mid-night is a decent idea, you'll get a lot of cheapskates who don't like buying VIP by their own money.

nice advice... IK cs is dying ... every game dies but as long as it is alive let us enjoy!