Why can I pick only two top players?

Because without a cap everyone picks the same five names. On historical data the five best players by form were used by 9 people out of 156 who played a season, and the strongest player was in almost every squad. The quota limits you to two players from the top 20% of the league, which widened the useful pool to 23 players at the cost of about 11% of the points. A player counts as top only after he has actually played at least two scored rounds; before that his place is set by his PBX rank.