Its the classic tradeoff between noise and sound quality.
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MOSFETs are similar to tubes in that they are operated by voltage swings instead of current. Tubes are iffy and require care and feeding in circuit to keep them relatively quiet and performing to spec. MOSFET's are simple and consistent and cheaper to install, so you get a more reliable product at lower cost, which is high on the list of design parameters. However, a tube input stage is always better than semiconductor, so that is why tube preamps will always be around.