I guess much that did the first client light weight was because it was developed by Ludvig Strigeus. He devloped the very light and popular uTorrent client. Very early for Spotify they bought uTorrent AB, not for the uTorrent client, but because they wanted his skills and he was the person that developed the first Spotify client. A few month later Spotify sold the uTorrent client to Torrent Inc.

Ludvig Strigeus is a skilled developer and have got many awards, like the Polhem Prize which is a Swedish award for a high-level technological innovation or an ingenious solution to a technical problem. If I recall correctly, he is very good at writing very small and optimized c++ programs, but the code is very hard for others to understand and maintain. So I guess when Spotify become bigger and more developer needed to work with the code they needed a more "main stream" solution and had to abandon the first client.