I don't think it has to be honest, and this is recency bias plus a hypothetic outlook on the future which doesn't always end up yielding what you would expect it to. I think its a bit unfair to think the league has become a two horse race when you consider that the only time Liverpool and Man city have finished first and second was 6 months ago. Prior to that you had top 2's of;
17/18: 1st Man city 2nd Man Utd
16/17: 1st Chelsea 2nd Spurs
15/16: 1st Leicester 2nd Arsenal
14/15: 1st Chelsea 2nd Man City
It's easy to jump on the bandwagon of "City and Liverpool are the two strongest sides to ever grace the premiership" but the reality is they are both very beatable, and Liverpool in particular are one or two injuries away from having some serious problems with the quality of their squad ( Van Dijk and any one of the front three). City have been beaten off Norwich and Wolves this season, and Liverpool will probably drop some unexpected points so i'd stop short of proclaiming them the top two just yet. After all, you only have to look at the table above to see just how crazy this league can be, and who's to say Leicester can't do it again? It's what is so spectacular about this league, the unpredictability, and even though on paper it would seem like Man City and Liverpool are streets above everyone else, it would take a sustained period of domination before the league could be considered a genuine two horse race every year.
Also, in recent times La Liga has begun to shake off its two horse race shackles, with Atletico Madrid now firmly established as a club that can genuinely compete with the big two. I'm not sure if your comparison was meant to reflect modern times or what the Spanish league was for so long, and even then there always comes a time when other teams step up (Valencia, Deportivo, Villareal)
The premier league is always a couple of moments away from turning a season on its head, lets hope that it continues to deliver on that front and brings a few people back down to earth.