The usual commercial problem with sleeper trains is that the vehicles carry significantly fewer passengers than a seated vehicle and often either cannot be used at all in daytime, or are stuck at the same capacity as they have overnight.
The standard capacities for a European train carriage (26m long, with a toilet) are 36 for a sleeper, 54 for a couchette (roughly hostel-equivalent accomodation), 57 for a first-class seated carriage and 76 for a second-class seated carriage. The proportions will be similar for buses.
This means that the sleeper cars have to earn enough money from fewer passengers doing one overnight journey to match several daytime journies for a seated car - but the sleeper is generally more expensive to buy and more expensive to run (overnight vs daytime pay for the driver, and usually attendants in addition to the guard/conductor). This means that fares have to be a lot higher for a sleeper to make a profit. Sleeper trains have been run by European railways as a loss-making public service, but are being wound down as railways become more commercial.
Sleeper buses are going to have a similar set of problems - they will cost more to buy than a conventional bus, and will carry fewer passengers, and will have higher running costs (at the very least, the bedding will need to be changed and washed) - and they can't be used for daytime journeys, so they will probably sit idle in the middle of the day.
That means they will need to charge a substantial fare premium over a seated coach to make a profit (at least double and more likely triple the fare). In countries where there are good alternatives like car rental or cheap flights, then this is likely to limit the market. Unless they can offer something distinctive (like an early morning arrival in a city where the airport is not allowed to have night landings), they may struggle to get passengers.
If there isn't an existing regulatory regime for getting sleeper buses approved, then the costs of getting the government to create one are likely to be too great for an operator to be prepared to pay - and note that some countries prohibit them entirely. Germany is a really important one, as many of the likely routes in Europe would run through Germany, but the German government banned sleeper buses in 2006.