It's because of its lineage, based on military aircraft. The Tu-134 provided a glass nose for a navigator position, providing them with a clear view of the ground ahead.
According to Tupolev Bureau, Tu-134 is an upgrade of Tu-124:
Tu-134 ... is an profound upgrade of Tu-124 with turbofan engines, arranged in the aft part of the fuselage.
Tu-124 from tupolev.ru
Tu-124, in its turn, was based on the Tu-104:
In 1958, designing of such passenger plane was started in the Tupolev Design Bureau. Tupolev’s proposal for creation of a new short-range passenger aircraft on the basis of the common aerodynamic and structural layout of Tu-104 has been worked out ...
Tu-104 from tupolev.ru
In turn, Tu-104 was based on the Tu-16 Badger, which meant that it inherited the glazed nose:
In the end of 1953, the Design Bureau’s leaders, headed by A.N. Tupolev, basing on the positive experience of developing, testing and beginning of serial production of Tu-16, proposed the idea of creating jet passenger aircraft on its basis to the leadership of the country.
On June 11, 1954 the Council of Ministers of the USSR issued a decree № 1172-516 on creation a long-range high-speed passenger aircraft designated Tu-16P .. . It was to be designed on the basis of Tu-16 airplane with two AM-3-200 engines.
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The glazed nose had a practical purpose- the navigator sat there (the aircraft needed a crew of three). The later versions of Tu-134 (Tu-134B) on the other hand, did away with the glazed nose.
Image from airliners.net
The Tu-334, on the other hand, had a completely new design, eliminating the glazed nose.