Why does the MAX30102 only work on a finger?
The article that Transistor pointed to goes into some detail on how pulse-oximeters work.

I won't try to reproduce all of it, but I'll summarize:

A pulse-oximeter counts the pulse by detecting changes in the light passed through the blood due to the increased volume of blood during each heart beat.

The article uses a pulse-oximeter with the emitter on one side of the finger and the detector on the other side - it works "through" the whole finger tip.

The MAX30102 is a reflective type - the emitter and detector are both on the same side.  That looks a little different, the principle is the same.

Now, as to why the MAX30102 works on your finger but not your chest:

Your finger tip has fairly decent blood flow.  There are arteries in the finger tip that transport a lot of blood.  The pulse-oximeter detects the blood flow in the arteries.

Your chest doesn't have much in the way of arteries.  The skin has a lot of capillaries, but no larger arteries.  There are arteries in the muscles beneath the skin, but they are too deep for the pulse-oximeter to "see."  

The LEDs have a limited light output (brightness.)  That limits how far the pulse-oximeter can "see" into or through your body.

The brightness is limited for a couple of reasons:


Low brightness = low power.  The MAX30102 is intended to be used in battery powered devices.  Low power means longer run time for the device  - or a smaller battery.
High power would be warm or even hot, possibly causing burns to the skin.


Short version:  

There's not enough blood flow through the skin of your chest for the MAX30102 to detect a pulse in it.



While the article linked above gets the technical details correct  (it correctly describes how a pulse-oximeter works,) it consistently gets the location of the arteries in the finger tip wrong.

It shows the arteries on the top of the finger (same side as the finger nail)  when in fact the arteries run along the "pad" side opposite the finger nail.

This drawing shows what I mean:



(Image taken from this paper on reattaching fingers.  Arteries in red, veins in blue.  Fingernail is drawn on the dorsal side.)

I figure a couple of doctors who reconnected arteries and veins on 24 fingers should have a good grasp of where the arteries and veins actually run.