It all comes down to cost. It is rather easy to build a large jet aircraft. To build an efficient and reliable large jet aircraft is not that easy. If you read the Wikipedia articles you linked carefully you find in the Il-96 article:
[...] the Il-96-300 had been deemed inferior to counterparts from Boeing and Airbus [...]
and in the Tu-204 article it is indirectly stated that the reliability of the engines isn't that great:
The PS-90A2 [engine] is expected [to have ...] life cycle cost saving of 35% over the original engine with a simultaneous increase in reliability in the 50 to 100% range.
Your next problem is line maintenance capabilities. That is more or less a chicken and egg problem. The Il-96 is a long range aircraft and you need maintenance on your destination. You need a certified mechanic, tools, and components. Component availability is another problem. If there is only a handful of aircrafts of one type there is no large spare supply around the world thus increasing the cost for the operator.
Another problem is that at least the Tu-204 is not fit for the current airspace structure:
The Tu-204SM [...] is an upgraded version of the Tu-204-100/300. [... T]here are numerous upgrades, largely aimed at meeting the current and near-future Russian and international standards [...].
And it requires a three man Cockpit Crew. Remember: The Tu-204 is an A321 sized aircraft.
The new cockpit features allow the Tu-204SM to be flown by a two-pilot crew (as compared with the three-pilot arrangement of the original Tu-204 series).
So far the improved Tu-204SM hasn't materialized so far and likely never will.
[... There are] currently no orders for the Tu-204SM, and Tupolev has frozen on development work on the aircraft and the UAC will withdraw it from their list of aircraft prices as soon as the Irkut MC-21 comes to the market.
As an airline has to keep the costs under control it is unwise to buy an aircraft which, from the start, is inferior to its competitors. It's a simple calculation: What costs more over the predicted lifespan. Initial costs (unit price) might be lower with one product but you have higher costs for fuel and maintenance. Add in some reliability problems and you get the costs incurred by those on top. Another product might have a higher unit price but the running costs are lower. Now do the math and choose whats best for your airline.
All quotes come from the Wikipedia articles on the Ilyushin Il-96 and the Tupolev Tu-204.