Why? Because English is very common already, and it seems an easy way to not only have usable words in two languages (English and Eng-Toki Pona) but also it would be useful to actually learn. (foreigners could _actually_ use this for real)
This is the biggest hurdle I have with made up languages, they have very little actual utility value.
As an afterthought, a simple charades like game where you have the limited vocabulary on a board, card or print out where people have to describe an event,book/story, object, movie, etc... using Eng-Toki Pona. Everyone could do it right now (even kids), and learn a new language at the same time. (one of the easiest ways to learn something is by making it fun and a game)
Edit: A reverse of this - Japanese-Toki Pona, all the same words, but in Japanese. Same with every other language (Russian, German, French, etc...). Then, after you learn one Toki derivative, you can easily pick up others. I figure it's possible you could learn the choppy/odd sentence building technique in your native language, and then add other languages later. Could also be fun as a group game once the English version gets easy. And all of these would be useful everywhere... not just speaking with other Toki Pona speakers.