'cause SSDs are not as reliable as they appeare to be to withstand the server-side workload. Enterprise-grade SSDs are significantly more expensive than consumer-grade ones. You are not looking at $1~2/GB price, but $10~20/GB. Given the capacity required for most use cases, SSDs are hardly good choice for critical servers as primary storage.
In addition, most RMDBS are optimized for mechanical disks. Optimization for SSDs becomes interesting only recently when the price of SSD drops to be barely reasonable.
In addition, most RMDBS are optimized for mechanical disks. Optimization for SSDs becomes interesting only recently when the price of SSD drops to be barely reasonable.
However, SSDs absolutely rocks as big cache.