To further elaborate: Because CDMA network carriers activate their phones on their end - as opposed to GSM carriers where the SIM card is the only real identification they can make, which is why you can swap your SIM card among different phones and they will just work as long as they are not locked from that carrier - Sprint and Verizon get to decide whether to activate any particular phone or not. When Verizon ran the number on your Evo, they'd see it was an Evo and probably refuse to activate it.
This is also what would likely prevent somebody from buying an Evo off-contract and trying to just activate it with Sprint to avoid signing up for the extra $10/month fee. Sprint will see that it's an Evo and require the fee in order to activate it.