Why do electricians prefer not to use the push-in connections on receptacles?
The sockets I've been using for the last 15 years have standard screw connection, holes behind that location where the screw pulls a plate against straight stripped wires and the push-in holes.

After you've encountered a few burnt sockets from using the push-in connection in the first couple outlets in a daisy chain, you start to realize something.


The straight connections behind the screw are mechanically sound and require only a wire stripper and a screwdriver. You don't really save much time using the push-in. 
Hoping and wishing that brass retains its spring pressure to maintain a tight, low resistance connection on the push-in connector after many heat cycles anneal it, doesn't make the connection mechanically sound. When the spring pressure relaxes as the metal loses its work hardening, you are left with a loose connection which heats, further anneals the brass spring and then proceeds to get hot enough to melt the supporting plastic shell.


Afraid to burn a house down during its lifetime? --OR-- Proud of sturdy craftsmanship that only takes a wire stripper and a screwdriver to make a solid, permanent connection?

Take your pick on how you word it, I'd rather err on the side of wearing the socket out from use, not having to replace it due to burning and melting from a loose connection.

Further observations:


On the mechanical connections: Brass has a similar coefficient of expansion as copper so is slower to loosen if tightened properly. This is one of the reasons why the old copper wire only sockets would fail with aluminum wire.
Due to daisy-chain failure, I once sectioned the side out of a new socket to verify "spring-loading" in the push-in contact. As copper wire is flexed, it work hardens. This allows the wire to put side pressure against the spring contact as the socket is pushed back into the box for mounting which can slightly unseat the connection. The lessened contact pressure causes resistance. Heat eventually anneals the spring causing it to lose its "tightening" ability leading to enough heat to melt the plastic supporting the contact. That observation is from disassembling a failed socket and the closeness that the hot conductor came to the others after burning/melting the support plastic was not good.
If you look up the receptacle specifications, the push-in is rated for 14 gauge wire. If your circuit breaker is 20 amp, you will be using 12 gauge wire and have to use screw head or straight in mechanical connections.