why sq-enix not add new servers or expand player host number capabilities thereof

[Llethander]

1.) They would have to purchase additional hardware, transport it to their data center (which can take days), set it up, test it, install the software on it, bring the other servers that would be connected to it offline, and then hope that once it was brought online it wouldn't cause any issues with the other servers.
2.) They work on a forecast. Every game has a surge of players around game/expansion launches and then typically die down within the first month. There's no sense in them setting up additional hardware only to have to scuttle it later.
3.) Reasons.