Why these territories iin Palestine are called West Bank and Gaza strip?

In 1948 according to UN mandate, Palestine was divided between lands occupied by recent Jewish settlers and the Palestinian Arabs who had been there for centuries. Palestine was an area under British control, bordering with Lebanon and Syria in the north, Jordan in the West and Egypt in the South. The dividing created a patchwork of small regions. The Arabs and (new) Israelis went to war and when the dust cleared the Palestinians were relegated to the West Bank of the river Jordan and the Gaza strip on the Mediteranean coast which was still part of Egypt. During the "six-day war" Israel seized the Golan Heights from Syria, the West Bank from Jordan and the Sinai peninsula from Egypt. The latter was returned to Egypt after a peace treaty and Gaza was given to the displaced Palestinians. Israel has occupied and has settlers in the West Bank despite objections from the UN.

The Palestinians are the inhabitants of Palestine which was stolen from them by illegal jewish immigrants from Europe after WW2. The jews formed terrorist gangs and drove the palestinians from their villages and massacred those who wouldn't go.
They then set up the State of Israel on this stolen land and the Palestinians were left with a small area on the coast which is called the Gaza Strip and an area to the west of the River Jordan called the West Bank.
Even on these small areas of land the jews have built armed settlements in defiance of the United Nations.
This is why most Palestinians now live in refugee camps.
The jews have the unconditional support of the USA.

Trader S that land belongs to the Jews it was theirs before there ever was a Palestine. In fact the name Palestine was given after the Jewish revolt in 132–135ad. Hadrian changed the name from Judea to Palestine and killed over 580,000 Jews to try to wipe them out. and in 629 they were betrayed by the Byzantine who offered them peace even though they granted asylum for siding with the Prussians and Finlay chased out of Israel all together. then the Byzantine empire lost it under the Muslim crusades so in fact the land was stolen. and the UN don't make me laugh they are just as anti Israel as Iran and Syria are

but that's why they are known as Palestinians

West Bank
The region did not have a separate existence until 1948–9, when it was defined by the Armistice Agreement between Israel and Jordan. The name "West Bank" was apparently first used by Jordanians at the time of their annexation of the region, and has become the most common name used in English and related languages. The term literally means 'the West bank of the river Jordan'; the Kingdom of Jordan being on the 'East bank' of this same river Jordan.

'Trader S' sums it up fairly well.
'Ryan s' got a bit hysterical - I think he must have meant 'Persians', not 'Prussians'! (They lived in northern Europe, centuries later!)
They are two bits of what was non-Jewish land for over 1800 years, after having been - arguably - Jewish for about 1500 years (if you are pro-Jewish!), and non-Jewish before that.
When the earlier wars after the creation of the State of Israel were finished, Israel had penned most of the non-Jews into these two areas, or driven them out. (What would be callled a "Diaspora", if done to Jews, but just an expulsion when done by them!)
They are the basis of the 'new' Palestinian State. One is on the West Bank of the Jordan River (+/- the City of Jerusalem!). The other is a narrow piece - a 'strip' - based on Gaza, which was Egyptian-controlled, at one time.

They're descriptive. A strip of land with the city of Gaza in it, and the west bank of the Jordan River.