Why output voltage becomes less when beyond resonating frequency?
In a common emitter amplifier, the gain will be given by the the collector impedance and load impedance:

\$A_v = \frac{Z_{collector}//R_L}{r_e}\$, where \$r_e = \frac{26mV}{I_E}\$

Given that, a tank circuit reaches its maximum impedance at its resonant frequency. In other words, \$Z_{collector}\$ is maximum at \$f_r\$.

For frequencies below or above \$f_r\$, its impedance is lower and decreases as long as the frequency moves further from \$f_r\$. So the parallel \$Z_{collector}//R_L\$, and thus Gain \$A_v\$, is low for any frequency different of \$f_r\$.

Therefore, you have a tuned amplifier which only amplifies signals of a certain frequency and "ignores" other frequencies.