Why is 433 MHz still used?
The regulations differ globally, so I'll just reference the Australian LIPD (Low Interference Potential Devices) class licence that I'm familiar with that is somewhat similar to other countries to give an example of some differences in a particular market:


All transmitters may use 433.05-434.79 MHz at 25 mW EIRP.
All transmitters may use 915-928 MHz at 3 mW EIRP.
Digital modulation transmitters may use 915-928 MHz at 1W EIRP but the radiated peak power spectral density in any 3 kHz is limited to 25 mW per 3 kHz. Also the minimum 6 dB bandwidth must be at least 500 kHz.


So 433 MHz may be used at 25mW with simple modulation schemes such as OOK / ASK / FSK making it a popular choice for keyless entry and other low data rate / cost sensitive applications. 

The 915 MHz band offers much greater bandwidth and power but the regulations essentially limit it to spread-spectrum operation at higher power. That tends to make it suitable for applications that are less cost sensitive but where higher bandwidth and/or range are required.