The reason that they couldn't do this faster is that until recently they were not in a legal position to ask for payment because they hadn't negotiated a deal with the organisations who collect royalties on behalf of songwriters - and it is important to understand that a songwriter isn't always the same person as the recording copyright owner and sometimes neither of those people are the artist, and in the early days particularly sometimes none of those people was the person uploading the music to SoundCloud. Legally making music available to stream is legally complex.

So they were in a bit of a chicken and egg situation - they needed to monetise the service but couldn't because they weren't legally able to, and to compound this they were sitting on a liability because technically they had been infringing the copyright of songwriters - but to do a deal to be legally able to monetise they had to be able to generate some sort of revenue stream to be able to get forward investment to settle the terms of that negotiation.

TL;DR - Soundcloud couldn't charge people without potentially being sued at some point about a copyright issue. To be able to charge people they needed to settle that problem and it seems like that all went on too long and now maybe they've hit a point where the model for monetising soundcloud isn't working as fast as they hoped.