Due to several reasons. First, it is a very popular and well known chip, an active product with large supporting code base and community of users. It is one with which I am acquainted quite well, and has proved itself in a number of other projects that I have made in the past. The programming tools for it are also easily available and don’t cost much. The chip itself is very robust and tolerates mistakes such as shorted pins or incorrect voltages, it has all the essentials without being bloated with rarely used stuff, and most importantly – comes in a DIP package.
As far as my knowledge goes, at the moment of building the ELLO 1A there was no other microcontroller with similar or better characteristics than PIC32MX170/270, also available on the market in through-hole package.
I have been appealing for some time now to Microchip for a release of the PIC32MZ version in DIP package. If that happens some day in the future, there could well be a more powerful version of ELLO for it too.