It's more than just Boca and River, the whole league is in serious trouble. In the last few seasons the quality has gotten a LOT worse. There are no good teams anymore, and with two 19 game seasons, it's very hard to be consistent, especially with the amount of parity there is. All it takes is a few losses at the beginning of the year and you lose all of your momentum and the season is done. The only teams that have been consistent in the last few years are Estudiantes and Velez, and to a lesser extent, Lanus. Every other team in Argentina has been very inconsistent, and you get tiny teams, such as Banfield, winning the championship.
I think the main reason has to do with salaries. The salaries are very low in Argentina and you see a lot of quality Argentine players leaving for much lesser leagues for a bigger paycheck (Sand going to Qatar, for example). The younger generation of Argentines aren't really into soccer, and almost the entire league is based in Buenos Aires. One city can't support a league, and attendance is declining a lot I think. But Brazil is a huge country that is doing very well economically with lots of big cities, so their clubs have lots of support (I think there are a lot of clubs in Brazil with 60,000 seater stadiums, and in Argentina, only River and Racing have stadiums of that size) and good financial backing. The salaries are way higher in Brazil, meaning less players leave, and the country is so big that they produce more good players.
