I blame france, the US, and our shitty kings
You fags had the gall to sink one of your own ships just to blame us, wage war against us and steal both cuba and the phillippines and everything else we had; why not interfere with all our other ex-colonies' governments too? That would indeed get them out of the picture as a menace, after all. Can't really blame you I guess, though you're still dicks for it.
France, specifically Napoleon, betrayed us and left us without control of our colonies, which were inspired both by the US and by France to start a revolution and independize.
Even before that, France made secret deals with the brits during the sucession war, basically castrating spain.
Hell, if it wasn't because of the EU we'd probably be like our colonies right now, since that's pretty much how we often were since the sucession war.
Bu the worst of all? Our shitty french kings, going all for centralization and fucking up our everything, as well as driving our colonies away from us. Surprise, during the few years spain had no king, we made a constitution and had improved our relation with our colonies, and were about to grant them their independence-with-alliance, ala british colonies. As soon as we kick napoleon out and the king is back he throws the constitution to the trash and starts warring the colonies to get them chained back to spain. And of course, let's keep milking Cuba to their last drop of blood! Which is pretty much what made the US start a war with us and get our last few colonies and sinking us into cultural depression.
Really, the reasons are just a whole mess of jerks, bad decisions and bad luck, otherwise known as History
If only we had stayed together, we could have had a chance at staying strong and developed. But not even the colonies themselves could manage to make their dream of an united american country come true, too much infighting.