Is it hard to create bass notes?
A marimba's resonator pipes get longer the lower the note. However, the marimba is an instrument that people play standing up, so there is only so much margin for changing the height of the tone plates. There are some instruments for which performers stand on tall platforms to play, but there is a limit even to this.
Therefore, a bit of ingenuity is required so that the long resonator pipes do not reach all the way to the floor. This is the reason that the ends of the pips are bent in L or U shapes. Because it increases the volume of the pipes without extending their lengths, sometimes resonator pipes are also made fatter or with wider ends.
In any case, the lower the note, the harder it is for the human ear to hear it. Therefore, the instrument must be made to produce louder volumes in the bass range. This becomes an issue when the range of an instrument is extended into lower notes.