Taildraggers are harder to control on the runway than tricycle types, the stick and rudder skill required is higher than that of other conventional tricycle aircraft which all helps to distract a student who is simply learning to fly. Orientation, RT and general airmanship will be put on the back burner until the student can control the aircraft to some degree sufficient to allow them to go solo, which will take longer. Lookout is the number one airmanship aspect and in taildraggers this is more difficult - it would help if you were first versed in the art of maintaining a good lookout before hopping into something less accomodating.
The time your instructor will need to spend with you on a taildragger at the ab-initio stage will be longer and sadly many schools do not allow adequate slot times for the extra briefing this would require, as a consequence safety standards would be in danger of lapsing below the norm.
But the main practical reason is that crosswind limits for taildraggers are much lower than conventional aircraft. In the olden days this was not a problem because airfields were exactly that - airfields. An aeroplane could always land and take-off into wind. Nowadays, runways dictate reduced take off and landing margins for taildraggers so you would get a higher percentage of lessons cancelled due to the wind whereas in a conventional tricycle type you'd be up there flying and learning. All this is not too much of a problem if you already know how to fly, but if you do not yet know how to fly then this all adds to the difficulty and your progress will suffer from the lack of continuity... it will take longer, more money and your training will have been disjointed leading to possible weak areas in basic skills which had you just done your time on a conventional type and then converted you'd have avoided. Need I go on? It's getting late.
VFE.