There are several reasons.
1. Joymax is greedy as fck and with true oldschool files stuff like Magic Pop, VIP, DG 12 Alchemy, Wheels, Devil Spirit etc. is missing. Meaning that a lot of items would be missing that would guarantee them cash. And they actually need those items. Without those items they would get way less money because there are not >40 servers with tons of real players. And even with an old school server they would not be able to get that many players back like they had back in 2007/2008/2009/2010.
2. They are stupid.
3. Silkroad is not there main focus anymore. A huge amount of there money now comes from mobile games (you might want to check this:
[Only registered and activated users can see links. Click Here To Register...]). It is just not worth to invest any more money in hosting new servers and actively developing the game. They probably get more money from licensing the files to JCP, Gamegami etc. instead of doing something on there own.
4. I guess it is way to late to bring back a lot of players to SRO. They should have released a server like that back in 2011 or 2012 when instead of Silkroad Revolution. Now most players are gone. Some grew up and left their basement. Others are playing other games, and others again are probably dead. Shit happens. Sure there would still be some old school players comming back after hearing about an official old school server. But it simply would not be worth the afford. Most of them would probably leave after a few days or weeks anyways when they noticed how boring SRO was even back then. Joymax probably knows that aswell.
5. They are stupid.
6. They dont care if char is real player or someones gold bot (player/company) as far he got SILVER VIP for 30$ and making their server crowded and full. One of the main reasons why nowadays server got like 20% real players and 80% goldbots.