Comment by nevdka

It wasn't just storage. In the 90s, most servers were single core. Databases already had network interfaces so they could be shared by non-web applications running on desktops, and so were the natural place to split a website over multiple machines. This turned into the received wisdom of always putting your database on a separate machine, despite processing power and storage latency improving so much since the 486 and pentium days that network overhead dominated response times.