Tricycle gear airplanes typically "rotate" for takeoff-- the nosewheel is lifted off the ground to place the aircraft in a nose-high pitch attitude, to increase the angle-of-attack of the wing.
If a tailwheel gear airplane were set up to have the fuselage basically horizontal on the ground, it too would need some way to "rotate"-- to lift the nose-- to increase the angle-of-attack of the wing for takeoff.1 But the tailwheel (touching the ground) would prevent this.
Being able to pitch up for take off. If the plane with a long tail
wheel strut tried to pitch up, the tailwheel would be pressed into the
ground. In this case couldn't they design some kind of suspension for
Yes that's basically it. You seem to be imagining some kind of elaborate suspension that would keep the fuselage level most of the time but would allow the pilot to still push the tail down for takeoff? That seems like a lot of weight to add to the tail of an airplane. CG issues can always be dealt with by lengthening the nose, but the increased pitch rotational inertia would likely be objectionable. Plus the added weight and complexity simply wouldn't be worth it.
Actually we could introduce a further nuance by asking "given that tailwheel airplanes typically lift the tail to take off, why not make the tailwheel long enough to support the aircraft in the usual take-off attitude, even if the fuselage is still somewhat nose-high rather than fully horizontal?"
The answer is that 1) there would be some extra weight and drag, and 2) it's advantageous to be able to land the aircraft in a pitch attitude and angle-of-attack where the wing is nearly completely stalled, which is a higher pitch attitude and angle-of-attack than would be safe for a normal takeoff.
- Unless of course the wings were attached to the fuselage at an unusually high "angle of incidence", so that they would be in a good position to create lift for takeoff even with the fuselage horizontal. This would have other disadvantages-- the fuselage would be flying in a nose-down attitude when the wings were flying at the relatively low angle-of-attack typically used for cruising flight. Of course there are exceptions to every rule, and there are some aircraft designed to be able to lift off the ground without "rotating" at all, so all the wheels lift off at the same time. Most (but not all) of these aircraft sit in a rather nose-high attitude on the ground. Others have minimal fuselages so fuselage drag is low regardless of pitch attitude. To the best of my knowledge none of these aircraft have either "tricycle" or "tailwheel" landing gear, though many aircraft with "tailwheel" landing gear can be safely "3-pointed" (a take off with all three wheels lifting the ground at the same time) in smooth conditions. Examples of aircraft designed to normally take off with all wheels lifting at the same time -- B-47, B-52, Aerianne Swift ultralight sailplane.