They are for all intents and purposes different instruments, the conventional guitar is more of a chordal instrument, the vast majority of chord types can be played using fingerings from standard tuning, all keys can also be traversed without altering tuning or anything else.
Notes on the guitar are generally fully fretted and it can be used as a chordal and a soloist instrument at the same time; in addition to which it has enough range for composition.
The lap guitar is an entirely different beast, notes are sounded using a slide of some type or other; chords are limited and are quite dependant on the tuning of the instrument.
In addition to the above; the lap guitar has a very unique and specific sound, which is not easy to emulate using a standard guitar and a slide.
Generally the standard style guitar is a lot more versatile in almost every respect; being able to fit your hand around the neck, apply accurate downward pressure using all fingers(and thumb), at speed, and being able to move around freely is the reason for this.