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Multiple reasons.
It's probably not going to be pristine and tidy for viewings with tenants in.
It's much harder to arrange viewings where you need to give tenants 24 hours notice.
You're more limited on dates if you're trying to tally up with giving tenants notice and them moving out (and you stand the risk of the property sitting empty if you don't get it right/sale falls through).
Possibly. But what to do in the meantime? Rent it out. Ok. It then becomes exponentially harder to sell.