The cost of virtualization
Virtualization is in the air around here, partly as a way of being environmentally friendly; fewer physical machines means less power and heating and so on. Now, I'd like to be environmentally friendly, but one of the issues with virtualization is that getting into it for anything important has significant startup costs.
One of the reasons we run different services on different machines to start with is for fault isolation, so we don't have all of our eggs in one basket. Virtualization reverses that; if you lose your host machine, you will lose any number of virtual machines. Thus if you are virtualizing important machines you need to be able to recover from this, which means at least two fairly beefy host machines and something approximating a SAN.
(This assumes that you aren't using virtualization just as a way of dealing with stupid software that demands to take over a whole machine.)
This issue makes it hard to just dabble your toes in virtualization. It's a serious investment and thus a serious commitment to get involved, which makes it a hard sell unless you are convinced that it is the way to go.
(This is especially so if you don't already have a SAN environment and a need for some big servers, so that if virtualization doesn't work out in practice you can't just use the hardware for something else.)
Note that in many cases you don't need expensive vendor software for live migration and failover and so on. Certainly in our environment it would be good enough just to bring the storage for the virtual machines up on another host machine and boot them back up, exactly as if we'd had a localized power failure with real hardware.
(Don't laugh. How many machines would you lose right now if one of your rack PDUs failed? If you can accept that, you can accept a virtualized version of it, especially as you can probably recover faster in the virtualized case. You might even be able to automate the recovery.)
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