Primarily for US-based players, but affects the rest of the world to some degree as well....... After Black Friday (BF) the two largest sites ((PokerStars (PS) and Full Tilt (FT)) were forced out of the US market and FT closed completely. Many, many players had money tied-up there which they have yet to recieve.
In the US, prior to BF, probably 90% of all players played on one of those 2 sites as their main, or only site. That number is a guess, but I'd say it's a fair approximation. Since BF there have been a number of other, smaller, sites to leave the US market as well. This leaves an ever-shrinking pool of what would have, pre-BF, been considered fringe sites as the only current option for US players. Service and reliability at these sites is shaky, to say the least.
Additionally, for US players, it is much harder now to get money on to, or off of, any poker site
. Again US post-BF regulations. Very long wait times to cash-out now, lots of hoops to jump through to even get money on to some sites, and the ever-nagging feeling that your chosen site could just disappear, a-la FT, and you may or may not ever see your money again.
Add all these things up and the result has been, for the US market, that a very high percentage of the casual players that existed before BF have simply stopped playing online. That leaves a smaller, more skilled, more dedicated group of players playing in the US now than pre-BF, hence the games have gotten considerably tougher. If you remove all or most of the casual players, pretty-much all that's left are those that take the game at least somewhat seriously who tend to just naturally be better players. They are about the only ones willing to put-up with all the onerous conditions that currently exist if you wish to play online poker in the US at the moment.