In Divrei ha'Yamim II, 24:26 it specifies that their mothers were Shim'as ha'Amonis and Shimris ha'Mo'avis. What is the reason?
Rashi (from Mechilta Beshalach, Amalek 1): It teaches that Hashem paid (punished) him through people like himself. Amonim and Mo'avim are ungrateful - they denied the good that Avraham did for Lot (from whom they descend) and fought the four kings to save Lot, and they 1 hired Bil'am to curse Avraham's seed. [Men from] these nations will pay Yo'ash, who denied the good that Yehoyada did for him, and killed his son Zecharyah.
The Torah mentions only Mo'av hiring Bil'am. Almost all Meforshim explain that Amon may not marry into Yisrael due to not offering us food when we passed by their land; also this shows ingratitude (PF). Malbim (Devarim 23:4) - at the time, Melech Mo'av was king also over Amon; it is as if also they hired Bil'am.
In Divrei ha'Yamim II, 24:25 it says that he was not buried in the kings' burial site. What is the reason?
Radak: The officers of Yehudah who supported him and enticed him to do evil were killed in the war. The nation recognized that this was vengeance from Hashem for his evils - idolatry and killing Zecharyah. Therefore they did this disgrace to him, not to bury him in the kings' burial site.
Malbim: Only Tzadikim were buried in the kings' burial site.
