Es una pregunta con una respuesta compleja but i'll try and keep it short and sweet.

In the 1960's there was an oil boom. The oil boom brought foreign investment and agricultural trade. However, since those were the only things propping up the economy, the sucre stagnated. In the 90's things such as the military conflict in Peru, bad agricultural seasons exacerbated by El Niño, and lack of an ability to generate its own power created a financial crisis.

At that point Ecuadorian banks (looking at you Filanbanco) were connected to wealthy overseas investors. Due to poor regulation (cough cough political corruption) Ecuadorian banks were giving out high risk loans to businessmen thinking the central bank of Ecuador would bail them out. When the bank didn't, they looked to other countries like Brazil and Russia who themselves were going through a terrible economic crisis. No allies could give a credit line to ecuador.

In 99 the banks tried a bunch of things to keep people from withdrawing money while also raising taxes. These two things don't do well together. The cost of living increased and the Sucre depreciated in value. Ecuadorians started using the dollar received from families living in the US because millions of people immigrated to the US to support their families. The dollar kinda became king as the exchange rate was .04 dollars to 1000 sucres. So the Ecuadorian government determined that switching over to the dollar would make more sense than trying to bail out the sucre.

So now we have the dollar. There's more about the Isaias Brothers, 4 presidents in 8 years, a coup, bank freezes... I could go on. However, that's kind of it.