Total Spencers made, including contract models, was under 90,000.
Henrys totalled only 14,000, BUT the '66 Winchester (Improved Henry) totalled 170,000 thtrough the end of production in 1898 and overlapped the '73 Winchester which was made until 1919 in numbers in excessof 720,000.
Lighter, faster to operate and reload, adequate power...all this led to the end of the Spencer.
Also the Spencer action wouldn't take the longer, more powerrful cartridges which came along in about 1876 and later.
Surplus Spencers were good guns and used extensively for many years. My grandfather had one and hunted with it until about 1900. (I wish I knew what had become of it.)